fix: normalize ISO date equality filters

[Aarkin7]

Related Issues

• fixes '==' and 'in' filters silently miss equivalent ISO timestamps (Z vs +00:00) #11962

Proposed Changes:

This fixes a gap in metadata filtering where ==, !=, in, and not in were still comparing ISO timestamps as raw strings. That meant equivalent values like 2025-02-03T12:45:46Z and 2025-02-03T12:45:46+00:00 could silently miss matching documents.

The fix normalizes date-like values for equality-style filters so they behave consistently with the ordering operators. I also added regression tests for both the filter helper and the InMemoryDocumentStore path, plus a release note.

How did you test it?

• Ran hatch run fmt on the touched files
• Ran hatch run test:unit test/utils/test_filters.py
• Ran targeted InMemoryDocumentStore filter tests
• Manually verified that == and in now return matches for equivalent ISO timestamps

Notes for the reviewer

The main logic change is in haystack/utils/filters.py.

The most useful review points are:

• the equality normalization helper in filters.py
• the new regressions in test/utils/test_filters.py
• the end-to-end store check in test/document_stores/test_in_memory.py

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes.
• I have added unit tests and updated the docstrings.
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I have documented my code.
• I have added a release note file, following the contributors guidelines.
• I have run pre-commit hooks and fixed any issue.

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[Aarkin7]

The remaining CI failures appear unrelated to this PR. They’re in test/components/generators/chat/test_openai_responses*.py with InputTokensDetails.cache_write_tokens validation errors. This PR only touches filter logic/tests and a release note.