feat(opensearch): Add support for nested fields in OpenSearchDocumentStore

[bogdankostic]

Related Issues

• fixes OpenSearch: Support nested Field Type for Correct Multi-Condition Array Filtering #3040

Proposed Changes:

Add support for OpenSearch nested field type in OpenSearchDocumentStore, enabling multi-condition filtering on array-of-objects metadata fields (e.g., filtering where refs.law == "bgb" AND refs.section == "1" matches within the same array element rather than across different elements).

Key changes:

• New nested_fields parameter on OpenSearchDocumentStore.__init__: accepts either explicit field names (e.g., ["refs", "tags"]) or ["*"] for auto-detection from list[dict] metadata values
• Filter translation: normalize_filters now wraps conditions targeting nested sub-fields in OpenSearch nested queries, grouping conditions on the same nested path for correct within-element matching
• Auto-detection mode (["*"]): detects list[dict] metadata fields during write_documents and dynamically updates the index mapping via put_mapping, supporting incremental detection across batches
• Existing index support: when the index already exists, nested fields are discovered from the live mapping at initialization
• Both sync and async code paths are updated

How did you test it?

• Unit tests for filter translation, nested path detection helpers, document store methods (serialization, default mappings, field detection, wildcard detection, incremental detection)
• Integration tests covering explicit nested fields (single filter, combined AND, mixed nested+flat, OR, different paths) and wildcard auto-detection (auto-detection, filtering, incremental detection)

Notes for the reviewer

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I 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:.

[github-actions]

Coverage report (opensearch)

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
integrations/opensearch/src/haystack_integrations/document_stores/opensearch
document_store.py					324-325, 338, 365-366, 375-384, 393-406, 421-423, 457-459, 643-652, 858-864, 888-894, 918-924, 958-964, 996-998, 1031, 1433-1437, 1538-1542, 1605-1615, 1716-1721, 1731-1736, 1752-1753, 1759-1761
filters.py					19-20, 28, 55
Project Total

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