fix: deduplicate embed_multi_with_metadata by content hash

[atharvasingh7007]

Summary

• deduplicate embed_multi_with_metadata() using existing content_hash values instead of existing item IDs
• align multi-item embedding behavior with the existing single-item embed() path
• add regression coverage for duplicate content submitted under a new ID

Testing

• .venv\Scripts\python -m pytest tests/test_embed.py -k embed_multi
• .venv\Scripts\python -m pytest tests/test_embed.py
• .venv\Scripts\python -m ruff check .
• .venv\Scripts\python -m mypy llm

Closes #1397