Fix an invalid quote escaping bug in f-string expressions.

[yilei]

Description

Fixes #3506

We can't simply escape the quotes in a naked f-string when merging string groups, because backslashes are invalid.

The quotes in f-string expressions should be toggled (this is safe since quotes can't be reused).

This fix also means implicitly concatenated f-strings with different quotes can now be merged or quote-normalized by changing the quotes used in expressions. e.g.:

         raise sa_exc.UnboundExecutionError(
             "Could not locate a bind configured on "
-            f'{", ".join(context)} or this Session.'
+            f"{', '.join(context)} or this Session."
         )

Checklist - did you ...

• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

[github-actions]

diff-shades results comparing this PR (f2801ef) to main (91e1e13). The full diff is available in the logs under the "Generate HTML diff report" step.

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