Split-attrs merge message

[pp-mo]

Closes #5585

During consideration of #5585 with @trexfeathers, we realised that although the way merge compares attributes is OK, the way it constructs a report of an attribute matching failure has some subtle problems in the "new world" of split attributes.
As stated in part of the testing code here,

        # Since Cube.attributes is now a "split-attributes" dictionary, it is now
        # possible to have "cube1.attributes != cube1.attributes", but also
        # "set(cube1.attributes.keys()) == set(cube2.attributes.keys())".
        # I.E. it is now necessary to specifically compare ".globals" and ".locals" to
        # see *what* differs between two attributes dictionaries.

This also affects the code where it works out which keys are different in existence ("cube.attributes keys differ: ") or value ("cube.attributes values differ for keys:") between two cubes whose attributes don't match.
Hence the changes here.

As so often, it was also useful to add some new tests to pin down existing behaviour,
When run against the prior feature-branch, behaviour looks like this :

$ pytest lib/iris/tests/test_merge_NEWSTUFF.py::TestCubeMerge__split_attributes__error_messages
================================================================================= test session starts ==================================================================================
platform linux -- Python 3.11.6, pytest-7.4.2, pluggy-1.3.0
rootdir: /net/home/h05/itpp/git/iris/iris_main
configfile: pyproject.toml
collected 6 items                                                                                                                                                                      

lib/iris/tests/test_merge_NEWSTUFF.py ..FF..                                                                                                                                     [100%]

======================================================================================= FAILURES =======================================================================================
______________________________________________ TestCubeMerge__split_attributes__error_messages.test_splitattrs_keys_local_global_mismatch ______________________________________________
iris.exceptions.MergeError: failed to merge into a single cube.
  cube.attributes keys differ: 'b'

During handling of the above exception, another exception occurred:

self = <iris.tests.test_merge_NEWSTUFF.TestCubeMerge__split_attributes__error_messages testMethod=test_splitattrs_keys_local_global_mismatch>

    def test_splitattrs_keys_local_global_mismatch(self):
        # Since Cube.attributes is now a "split-attributes" dictionary, it is now
        # possible to have "cube1.attributes != cube1.attributes", but also
        # "set(cube1.attributes.keys()) == set(cube2.attributes.keys())".
        # I.E. it is now necessary to specifically compare ".globals" and ".locals" to
        # see *what* differs between two attributes dictionaries.
>       self._check_merge_error(
            attrs_1=CubeAttrsDict(globals=dict(a=1), locals=dict(b=2)),
            attrs_2=CubeAttrsDict(locals=dict(a=2)),
            expected_message="cube.attributes keys differ: 'a', 'b'",
        )

lib/iris/tests/test_merge_NEWSTUFF.py:1179: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
lib/iris/tests/test_merge_NEWSTUFF.py:1139: in _check_merge_error
    with self.assertRaisesRegex(
E   AssertionError: "cube.attributes keys differ: 'a', 'b'" does not match "failed to merge into a single cube.
E     cube.attributes keys differ: 'b'"
________________________________________ TestCubeMerge__split_attributes__error_messages.test_splitattrs_keys_local_match_masks_global_mismatch ________________________________________
iris.exceptions.MergeError: failed to merge into a single cube.
  cube.attributes values differ for keys: 

During handling of the above exception, another exception occurred:

self = <iris.tests.test_merge_NEWSTUFF.TestCubeMerge__split_attributes__error_messages testMethod=test_splitattrs_keys_local_match_masks_global_mismatch>

    def test_splitattrs_keys_local_match_masks_global_mismatch(self):
>       self._check_merge_error(
            attrs_1=CubeAttrsDict(globals=dict(a=1), locals=dict(a=3)),
            attrs_2=CubeAttrsDict(globals=dict(a=2), locals=dict(a=3)),
            expected_message="cube.attributes values differ for keys: 'a'",
        )

lib/iris/tests/test_merge_NEWSTUFF.py:1186: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
lib/iris/tests/test_merge_NEWSTUFF.py:1139: in _check_merge_error
    with self.assertRaisesRegex(
E   AssertionError: "cube.attributes values differ for keys: 'a'" does not match "failed to merge into a single cube.
E     cube.attributes values differ for keys: "
=================================================================================== warnings summary ===================================================================================
lib/iris/tests/test_merge_NEWSTUFF.py::TestCubeMerge__split_attributes__error_messages::test_keys_differ__multiple
  <frozen importlib._bootstrap>:241: RuntimeWarning: numpy.ndarray size changed, may indicate binary incompatibility. Expected 16 from C header, got 96 from PyObject

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=============================================================================== short test summary info ================================================================================
FAILED lib/iris/tests/test_merge_NEWSTUFF.py::TestCubeMerge__split_attributes__error_messages::test_splitattrs_keys_local_global_mismatch - AssertionError: "cube.attributes keys differ: 'a', 'b'" does not match "failed to merge into a single cube.
FAILED lib/iris/tests/test_merge_NEWSTUFF.py::TestCubeMerge__split_attributes__error_messages::test_splitattrs_keys_local_match_masks_global_mismatch - AssertionError: "cube.attributes values differ for keys: 'a'" does not match "failed to merge into a single cube.
======================================================================== 2 failed, 4 passed, 1 warning in 2.18s ========================================================================

NOTE: it was also noted in the above that sometimes the list of differing keys appears in a different order, due to use of set logic.
I have also now added sorted to fix that in a couple of places

---

Consult Iris pull request check list

[pp-mo]

@trexfeathers for your consideration ?
Enjoy !!

[codecov]

Codecov Report

Attention: 37 lines in your changes are missing coverage. Please review.

Comparison is base (0b54d58) 89.25% compared to head (58d48db) 89.46%.
Report is 143 commits behind head on FEATURE_split_attrs.

Files	Patch %	Lines
lib/iris/fileformats/netcdf/saver.py	91.06%	12 Missing and 4 partials ⚠️
lib/iris/fileformats/netcdf/_thread_safe_nc.py	79.48%	7 Missing and 1 partial ⚠️
lib/iris/__init__.py	70.00%	2 Missing and 1 partial ⚠️
lib/iris/analysis/__init__.py	96.80%	1 Missing and 2 partials ⚠️
lib/iris/fileformats/netcdf/_dask_locks.py	92.68%	2 Missing and 1 partial ⚠️
lib/iris/_concatenate.py	98.57%	0 Missing and 1 partial ⚠️
lib/iris/cube.py	99.10%	0 Missing and 1 partial ⚠️
lib/iris/fileformats/name_loaders.py	0.00%	1 Missing ⚠️
lib/iris/quickplot.py	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##           FEATURE_split_attrs    #5590      +/-   ##
=======================================================
+ Coverage                89.25%   89.46%   +0.21%     
=======================================================
  Files                       88       90       +2     
  Lines                    22197    22611     +414     
  Branches                  4858     5441     +583     
=======================================================
+ Hits                     19811    20228     +417     
+ Misses                    1641     1638       -3     
  Partials                   745      745              

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