fix: apply FilterPolicy.MERGE correctly in in-memory retrievers

[Aarkin7]

Related Issues

• fixes FilterPolicy.MERGE silently drops init filters in InMemoryBM25Retriever and InMemoryEmbeddingRetriever #12065

Proposed Changes:

InMemoryBM25Retriever and InMemoryEmbeddingRetriever both documented FilterPolicy.MERGE, but for the common comparison-filter shape they were effectively behaving like REPLACE: the runtime filter overwrote the init-time filter instead of narrowing the result set.

This PR switches both retrievers to the shared apply_filter_policy() helper in their sync and async paths, so init and runtime filters are combined correctly. It also adds focused regression tests for the broken behavior in both retrievers and includes a release note.

How did you test it?

• Ran hatch run test:unit test/components/retrievers/test_in_memory_bm25_retriever.py test/components/retrievers/test_in_memory_embedding_retriever.py
• Ran hatch run test:unit test/document_stores/test_filter_policy.py test/components/retrievers
• Ran hatch run test:types
• Manually verified the important cases against in-memory BM25 and embedding retrieval:
  • MERGE combines init and runtime filters
  • REPLACE still preserves the old override behavior
  • same-field MERGE still follows the helper's existing warning/runtime-wins behavior

Notes for the reviewer

• The production change is intentionally small and only touches the two in-memory retrievers that implement filter_policy.
• Wrapper retrievers were not changed because they just pass filters through to the underlying retriever.
• The new tests cover both sync and async execution paths.

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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Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/components/retrievers/in_memory
bm25_retriever.py
embedding_retriever.py
haystack/document_stores/in_memory
document_store.py
haystack/document_stores/types
filter_policy.py
Project Total

This report was generated by python-coverage-comment-action

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Project	Deployment	Actions	Updated (UTC)
haystack-docs	Ready	Preview, Comment	Jul 20, 2026 8:12am

[davidsbatista]

looks good!