[SunSi12138]

Problem

A timed ServerStreaming call observes head-Request emission, but the emission wait uses CancellationToken.None. If the transport write stalls after queue admission, PendingRequestTable can already select DeadlineExceeded (and complete the user-visible dispatcher) while ServerStreamingInvoker remains parked in the framework-task supervisor until the transport recovers.

Change

• add a small PendingCallTerminalSignal completion-observer wrapper that mirrors the terminal already selected by PendingRequestTable into a framework-only cancellation token;
• keep the existing completion observer chained through the wrapper, so endpoint/admission accounting still observes the same terminal exactly once;
• use that token only for timed ServerStreaming head-Request emission waits;
• dispose the signal source as soon as emission completes normally, so normal stream lifetime does not retain an extra CTS;
• leave publication and terminal arbitration unchanged: PendingRequestTable remains authoritative, an accepted Request remains transport-owned, and the emission cancellation cannot replace the winning terminal reason.

The signal is intentionally not a producer token: ServerStreaming has no local producer. It represents only pending-call terminal lifetime.

Regression coverage

Adds a deterministic manual-clock/transport-stall test that:

1. stalls the transport after the ServerStreaming Request has been admitted;
2. advances the pending deadline;
3. verifies the consumer observes DeadlineExceeded;
4. verifies ServerStreamingInvoker leaves the framework supervisor before the transport stall is released; and
5. releases the transport and verifies the already-admitted Request is emitted before its matching Cancel.

Local SDK execution is intentionally not used; validation is through the repository's existing GitHub Actions workflows.

Fixes #687