fix(testing): pytest.approx returns a clearer error mesage when comparing mappings with different keys

[cmnemoi]

Hello,

This PR fixes an issue with pytest.approx where the error message was not clear when comparing mappings with different keys. Previously, the comparison would proceed and report -inf mismatches on values or a KeyError, leading to confusing error messages.

The fix adds an early check in ApproxMapping to compare the sets of keys between the expected and actual mappings. If they differ, it returns a clear error message indicating the different keys.

This closes #13816.

Checklist

• I have included new tests or updated existing ones where applicable.
• I have created a new changelog file in the changelog folder, named <ISSUE NUMBER>.<TYPE>.rst.
• I have added myself to AUTHORS in alphabetical order.
• I have added text like "closes #XYZW" to the PR description.
• I have allowed maintainers to push and squash when merging my commits.
• I have included documentation when adding new features. (Not applicable here – this is a bug fix.)