feat(gax): report response status codes on traced calls

[shivanee-p]

Pass through the status code to spans. rpc.response.status_code holds the gRPC status name on both transports, since gax resolves it everywhere. grpc.response.status_code mirrors it on gRPC spans, and http.response.status_code carries the received HTTP status on fallback spans.

The HTTP status is plumbed from the fetch response through decodeResponse onto GoogleError.httpStatusCode, and is absent when no response arrived at all, such as an expired deadline.

Success is inferred rather than observed. The unary path never surfaces a status object to gax, so a call that reported no error is recorded as OK, and 200 on the fallback.

Adds OtelHarness.assertResponseStatus, which derives the expected attribute shape from the span's own gcp.method.type and asserts both the presence of the attribute that applies and the absence of the one that does not, so a test cannot assert a combination the tracer should never produce.

[gemini-code-assist]

Code Review

This pull request enhances error reporting and telemetry in the REST fallback transport by capturing and recording the actual HTTP response status code. It introduces an 'httpStatusCode' property to 'GoogleError' and updates the OpenTelemetry tracing helper ('TracerHelper') to extract and attach 'rpc.response.status_code', 'grpc.response.status_code', and 'http.response.status_code' attributes to spans. Comprehensive unit tests have also been added to verify these changes. I have no further feedback to provide as the implementation is robust and well-tested.