feat: make asyncio extra optional

[theanupllm]

Summary

• Remove hard dependency on pinecone[asyncio] for sync‑only users; require it only when async APIs are used.
• Provide clear, actionable ImportErrors for async paths when the extra isn’t installed.

Changes

• Async optionalization and lazy imports:
  • vectorstores: lazy‑import PineconeAsyncio; in async_index raise ImportError with guidance if missing; sync paths unchanged.
  • embeddings: lazy‑import PineconeAsyncio; async_client raises ImportError with guidance if missing.
  • rerank: lazy‑import PineconeAsyncio; _get_async_client raises ImportError; public arerank logs and returns [] when extra is missing.
• Packaging updates (libs/pinecone/pyproject.toml):
  • Base dep now pinecone>=6,<8 (no asyncio extra by default).
  • Add optional extra: asyncio = ["pinecone[asyncio]>=6,<8"].
  • Ensure CI retains async coverage by adding pinecone[asyncio] to test and test_integration groups.
• Tests:
  • Add tests/unit_tests/test_optional_asyncio.py verifying:
    • Sync vector store and embeddings work without asyncio extra.
    • Async vector store/embeddings raise ImportError.
    • Reranker _get_async_client() raises ImportError; arerank() returns [] and logs guidance.

Developer Notes

• Sync usage requires no change.
• For async usage, consumers should install one of:
  • pip install langchain-pinecone[asyncio] (package extra)
  • or ensure pinecone[asyncio] is present in their environment.

Fixes

• Addresses: pinecone[asyncio] extra shouldn't be required dependency #37 (pinecone[asyncio] extra shouldn’t be required dependency).