fix: correctly represent yaml dumped multiline strings

Add a representer as we do for other craft applications to correctly
dump multi line strings.

craft_application/models/base.py

@@ -16,7 +16,7 @@
 """Base pydantic model for *craft applications."""
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, Type, cast
 
 import pydantic
 import yaml
@@ -34,6 +34,17 @@ def _alias_generator(s: str) -> str:
     return s.replace("_", "-")
 
 
+# pyright: reportUnknownMemberType=false
+# Type of "represent_scalar" is "(tag: str, value: Unknown, style: str | None = None)
+# ->
+# ScalarNode" (reportUnknownMemberType)
+def _repr_str(dumper: yaml.Dumper, data: str) -> yaml.ScalarNode:
+    """Multi-line string representer for the YAML dumper."""
+    if "\n" in data:
+        return dumper.represent_scalar("tag:yaml.org,2002:str", data, style="|")
+    return dumper.represent_scalar("tag:yaml.org,2002:str", data)
+
+
 class CraftBaseConfig(pydantic.BaseConfig):  # pylint: disable=too-few-public-methods
     """Pydantic model configuration."""
 
@@ -83,4 +94,7 @@ def from_yaml_file(cls, path: pathlib.Path) -> Self:
     def to_yaml_file(self, path: pathlib.Path) -> None:
         """Write this model to a YAML file."""
         with path.open("wt") as file:
+            yaml.add_representer(
+                str, _repr_str, Dumper=cast(Type[yaml.Dumper], yaml.SafeDumper)
+            )
             yaml.safe_dump(self.marshal(), file)

tests/unit/models/project_models/full_project.yaml

@@ -1,6 +1,8 @@
 base: core24
 contact: [email protected]
-description: A fully-defined craft-application project. (description)
+description: |
+  A fully-defined craft-application project.
+  With more than one line.
 issues: https://github.com/canonical/craft-application/issues
 license: LGPLv3
 name: full-project

tests/unit/models/test_project.py

@@ -15,6 +15,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 """Tests for BaseProject"""
 import pathlib
+from textwrap import dedent
 from typing import Optional
 
 import pytest
@@ -44,14 +45,19 @@
     issues="https://github.com/canonical/craft-application/issues",
     source_code="https://github.com/canonical/craft-application",  # pyright: ignore[reportGeneralTypeIssues]
     summary="A fully-defined craft-application project.",  # pyright: ignore[reportGeneralTypeIssues]
-    description="A fully-defined craft-application project. (description)",
+    description="A fully-defined craft-application project.\nWith more than one line.\n",
     license="LGPLv3",
     parts=PARTS_DICT,
 )
 FULL_PROJECT_DICT = {
     "base": "core24",
     "contact": "[email protected]",
-    "description": "A fully-defined craft-application project. (description)",
+    "description": dedent(
+        """\
+        A fully-defined craft-application project.
+        With more than one line.
+    """
+    ),
     "issues": "https://github.com/canonical/craft-application/issues",
     "license": "LGPLv3",
     "name": "full-project",