test(showcase): add IT against showcase for ResumableUploadCallable

Add integration tests against local GAPIC Showcase server for
ResumableUploadCallable, validating:
1. Single-chunk upload happy path.
2. Multi-chunk upload with multiple intermediate chunks and final chunk.
3. 0-byte upload and exact-chunk uploads with separate 0-byte finalize request.
4. Future cancellation and timeout handling.
5. Convenience futureCall(request, stream) overload.
6. Per-request ApiCallContext extra header overrides.
7. Stub-level and per-request ResumableUploadCallSettings overrides.
8. Stub-level and per-request global timeout watchdog triggering DeadlineExceededException.

Author: whowes