feat(supabase): add async support to SupabaseGroongaDocumentStore and SupabaseGroongaBM25Retriever

[Aftabbs]

Summary

• Adds warm_up_async() and a full set of *_async() methods to SupabaseGroongaDocumentStore using supabase-py's AsyncClient and acreate_client
• Fixes SupabaseGroongaBM25Retriever.run_async(), which was a stub that delegated to the synchronous run() instead of using real async I/O
• Adds 14 new unit tests for the async path across both classes

Closes #3379

Root Cause

supabase-py ships two parallel clients: create_client (sync) and acreate_client (async). The original implementation only wired up the sync client. run_async() on the retriever simply called self.run(), blocking the event loop on every invocation — making AsyncPipeline usage effectively synchronous.

Changes

File	What changed
groonga_document_store.py	Added _async_client field; warm_up_async(), _setup_table_async(), and async counterparts for all public methods (count_documents_async, filter_documents_async, write_documents_async, delete_documents_async, delete_all_documents_async, delete_by_filter_async, update_by_filter_async, _groonga_retrieval_async)
groonga_bm25_retriever.py	Fixed run_async() to call _groonga_retrieval_async(); removed stale docstring note saying async is unsupported
test_groonga_document_store.py	Added TestDocumentStoreAsync class with 12 async unit tests using AsyncMock for the async client
test_groonga_retriever.py	Updated test_run_async to verify _groonga_retrieval_async is actually awaited (not the sync path)

Testing

• Existing sync tests pass (no sync code was changed)
• New async tests: TestDocumentStoreAsync — 12 tests covering warm_up, count, filter, write (OVERWRITE/SKIP/FAIL policies), delete, delete_all, retrieval, and uninitialized-client guard
• Retriever test: test_run_async_uses_async_retrieval — asserts _groonga_retrieval_async is awaited
• ruff check passes with no errors

Impact

Users running SupabaseGroongaBM25Retriever inside an AsyncPipeline now get true non-blocking I/O instead of a blocking call that defeats the purpose of async execution.