fix(testing): pytest.approx correctly take account Mapping keys order to compare them

[cmnemoi]

Hello,

This PR fixes an issue with pytest.approx where the error message incorrectly reported all elements as mismatched when comparing mappings with different key orders, even when only some values differed.

The original code paired values by position rather than by key. This caused incorrect mismatch reporting when dictionary keys were in different orders.

This closes #12444.

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.)