feat(agents-api): Add ``metadata`` field to ``DocReference`` and retrieve it in the query

[Ahmad-mtos]

Added metadata to DocReference in typespec, and fetched the metadata in search_docs_by_text and search_docs_by_embedding

Closes #814

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[!IMPORTANT] Add metadata field to DocReference and update search functions and OpenAPI schema to handle it.

• Behavior:
  • Add metadata field to DocReference in Docs.py and models.tsp.
  • Update search_docs_by_text.py and search_docs_by_embedding.py to retrieve metadata from documents.
• OpenAPI:
  • Update openapi-1.0.0.yaml to include metadata in DocReference schema.
• Tests:
  • Modify tests in test_docs_queries.py to check for metadata in search results.

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