fix: preserve cloud storage URIs in LanceDB vector store config validation

[ohharsen]

Description

_validate_vector_store_db_uri() unconditionally calls Path(db_uri).resolve() on the LanceDB db_uri, which destroys cloud storage URIs. For example, gs://my-bucket/indexes/lancedb gets resolved to /current/working/dir/gs:/my-bucket/indexes/lancedb — stripping the double slash from the scheme and prepending the CWD. This silently breaks LanceDB's native support for GCS, S3, and Azure cloud storage backends.

Related Issues

N/A — discovered while using LanceDB with a gs:// URI for Google Cloud Storage.

Proposed Changes

• graphrag/config/models/graph_rag_config.py: Skip Path(db_uri).resolve() when db_uri starts with a recognized cloud storage scheme (gs://, s3://, az://, abfs://). Local file paths continue to be resolved as before.

 def _validate_vector_store_db_uri(self) -> None:
     """Validate the vector store configuration."""
     store = self.vector_store
     if store.type == VectorStoreType.LanceDB:
         if not store.db_uri or store.db_uri.strip == "":
             store.db_uri = graphrag_config_defaults.vector_store.db_uri
-        store.db_uri = str(Path(store.db_uri).resolve())
+        if not store.db_uri.startswith(("gs://", "s3://", "az://", "abfs://")):
+            store.db_uri = str(Path(store.db_uri).resolve())

Checklist

• I have tested these changes locally.
• I have reviewed the code changes.
• I have updated the documentation (if necessary).
• I have added appropriate unit tests (if applicable).

Additional Notes

Reproduction steps:

1. Configure VectorStoreConfig with a cloud URI: db_uri="gs://my-bucket/path/to/lancedb"
2. Pass it to GraphRagConfig(vector_store=...)
3. Observe that config.vector_store.db_uri is now a local filesystem path (e.g. /cwd/gs:/my-bucket/path/to/lancedb)
4. local_search and drift_search fail with AttributeError: 'LanceDBVectorStore' object has no attribute 'document_collection' because lancedb.connect() can't find any tables at the mangled path

This also affects @validate_call on local_search/drift_search, which re-validates the config (creating a Pydantic copy), so even manually overriding db_uri after initialization doesn't work.

LanceDB has supported cloud storage URIs natively since v0.6+ (docs). This one-line fix enables that integration to work through GraphRAG's config layer.