feat: send X-Weaviate-Client-Integration telemetry header

Weaviate now tracks client integration usage via the
X-Weaviate-Client-Integration header (weaviate/weaviate#10535). Tag the
connection with "langchain/<version>" so LangChain adoption shows up in
Weaviate telemetry.

The header is registered best-effort in WeaviateVectorStore.__init__ via
the weaviate-client public integrations API (covers both HTTP and gRPC).
It never breaks construction if the client lacks the API or registration
fails. The version is the langchain-weaviate package version, read at
runtime via importlib.metadata with an "unknown" fallback.