[Bug]: ClusterStorage.get return annotations exclude valid None results

System Info

Static Python API contract in tensorrt_llm/serve/cluster_storage.py on current main.

Reproduction

ClusterStorage.get() and its concrete implementations are annotated as returning str, but their existing behavior returns None for missing or expired keys. The etcd implementation can also return None through @handle_etcd_error(return_on_error=None).

Affected declarations include the shared ClusterStorage.get() contract and the HTTP server/client and etcd implementations.

Expected behavior

Annotate the shared declaration and all concrete get() implementations as str | None, matching the existing runtime contract. This should be a type-only cleanup with no behavior change.

Actual behavior

The annotations currently promise str even though None is an established result for missing, expired, and some error cases.

Additional notes

Surfaced while reviewing #17771. Kept separate because the mismatch predates that PR and is broader than its empty-etcd-value fix.

Start in tensorrt_llm/serve/cluster_storage.py and inspect the shared ClusterStorage.get() contract plus the HTTP server/client and etcd implementations mentioned in the issue. Update only the return annotations so each accepts None, then verify that the declarations consistently use str | None and that runtime behavior is unchanged.