test: make retry observation waits terminal-aware

[SunSi12138]

Summary

Follow-up for the retry-liveness flake tracked in #387.

#637 already landed the root-cause isolation for SharpLinkClientRetryBehaviorTests.IdempotentUnaryShouldRetryRemoteUnavailableAndExposeResponseObservation by removing the unrelated one-second System request timeout from this response-observation/retry-policy test.

This PR closes the remaining diagnostic/liveness gap in the same test:

• both expected Request emissions were previously awaited through unbounded WaitForSentPacket(Request) calls;
• if the logical invocation becomes terminal before either expected Request can be emitted, the transport wait has no knowledge of that terminal state and can mask the real failure until the outer TUnit watchdog fires;
• race each Request observation against the invocation itself;
• if the invocation wins, await it immediately so the real exception is surfaced; if it completed successfully unexpectedly, fail with the exact emission phase.

The test still requires both the first Request and the retry Request to be emitted before injecting the successful second-attempt response. No retry, deadline, transport, or production behavior is weakened.

Validation

• Static analysis and diff inspection only.
• Branch is one commit ahead of dev and changes only test/SharpLink.UnitTests/Client/SharpLinkClientRetryBehaviorTests.cs.
• No test/job/workflow reruns were manually triggered.

Refs #387
Follow-up to #637