Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@modelexpress_server/src/k8s_types.rs`:
- Around line 94-96: The WorkerStatus field transfer_engine_session_id
(serialized as transferEngineSessionId) was added in Rust but is missing from
the CRD OpenAPI schema; update the workers array item schema in
crd-modelmetadata.yaml to add a transferEngineSessionId property (string,
optional/nullable) under the WorkerStatus properties so the CRD matches the
WorkerStatus struct, include an appropriate description, and do not mark it as
required.

In `@modelexpress_server/src/metadata_backend/kubernetes.rs`:
- Around line 263-274: WorkerStatus.nixl_metadata is currently serialized as
base64 but deserialization (where ModelMetadata.status.workers[].nixlMetadata is
read) assumes base64 and will fail for previously persisted non-base64 entries;
add a compatibility deserializer: annotate the nixl_metadata field with
serde(deserialize_with = "deserialize_nixl_metadata"), implement a
deserialize_nixl_metadata function that tries base64::decode and deserializes
the decoded bytes, and if base64 decoding fails or the decoded bytes are
invalid, fall back to interpreting the input as a raw UTF-8/JSON string (or
already-encoded form) and produce the same internal string representation;
ensure the deserializer returns the same type used by WorkerStatus.nixl_metadata
and include unit tests covering both legacy and base64 formats.

---

Nitpick comments:
In `@modelexpress_server/src/metadata_backend/kubernetes.rs`:
- Around line 263-269: The serialization flattener in kubernetes.rs (the match
assigning nixl_metadata and transfer_engine_session_id) loses the discriminator
because Nixl(vec![]) and None both become nixl_metadata = "" and
transfer_engine_session_id = None; modify this match to also emit an explicit
backend kind/discriminator (e.g., backend_kind = "Nixl" | "TransferEngine" |
"None") when constructing the flattened CRD payload so the reader can
reconstruct the original BackendMetadataRecord; set backend_kind alongside
nixl_metadata and transfer_engine_session_id in the same match arm and ensure
the reader/deserialize path consumes that discriminator to restore the correct
enum variant.

In `@modelexpress_server/src/state.rs`:
- Around line 239-266: Update the test_worker_record_conversion unit test to
include and verify the new TransferEngine branch: construct a WorkerMetadata
that sets backend_metadata to the TransferEngine variant (with a
transfer_engine_session_id value), convert it to WorkerRecord via
WorkerRecord::from(...) and back into WorkerMetadata via Into, and assert that
worker_rank, backend_metadata (specifically the TransferEngine session id), and
tensors round-trip correctly; reference the existing
test_worker_record_conversion, WorkerMetadata, WorkerRecord, and
BackendMetadataRecord symbols when adding the new case so the backwards-compat
wrapper will catch regressions.