fix(test): fix TChannel span test flakiness by awaiting async spans and explicitly setting worker tracer

[Copilot]

Tests testStartWorkflowTchannel and testSignalWithStartWorkflowTchannel were failing because (1) TChannel finishes client spans asynchronously in response callbacks — spans may not be visible the moment the calling thread resumes — and (2) the worker's tracer was not reliably inheriting mockTracer from service options due to instanceof NoopTracer check fragility from transitive opentracing-noop version conflicts.

What changed?

• FakeWorkflowServiceRule: Added awaitSpan(operationName) — polls finishedSpans() up to 5s to handle TChannel's async span completion in unit tests
• WorkflowClientInternalTest: Switched assertions to use awaitSpan instead of reading finishedSpans() directly
• StartWorkflowTest (docker integration tests):
  • Added awaitSpans() with 30s timeout in testStartWorkflowHelper and testSignalWithStartWorkflowHelper to handle async span visibility
  • Explicitly pass mockTracer to WorkerOptions when shouldPropagate=true rather than relying on Worker.java's instanceof NoopTracer fallback, which can silently fail when classpath contains mismatched opentracing-noop versions

// Before: relied on Worker.java's instanceof NoopTracer fallback
worker = workerFactory.newWorker(TASK_LIST, WorkerOptions.newBuilder().build());

// After: explicitly propagate mockTracer when tracing is expected
WorkerOptions workerOptions = shouldPropagate
    ? WorkerOptions.newBuilder().setTracer(mockTracer).build()
    : WorkerOptions.newBuilder().build();
worker = workerFactory.newWorker(TASK_LIST, workerOptions);

Why?

TChannel spans are finalized in a response callback thread, not the caller thread. Tests that read finishedSpans() synchronously after a TChannel call can see incomplete span state. Additionally, Worker.java's automatic tracer propagation via instanceof NoopTracer is fragile under dependency version conflicts between tchannel-core's transitive opentracing and the test's opentracing-mock.

How did you test it?

Changes validated against the CI test run that exposed the failures (job 94609211566).

Potential risks

None — test-only changes.

Release notes

None.

Documentation Changes

None.