test(server): stabilize watch span cancellation test

[pimlock]

Summary

Stabilize watch_producer_releases_request_span_when_client_disconnects by giving the detached producer and tracing exporter a five-second bounded observation window instead of 200 ms. The wait polls every 10 ms and matches the existing timeout pattern used by neighboring asynchronous server tests.

Observed failures:

• linux-arm64 CI failure
• linux-amd64 CI failure

Related Issue

No issue required: this is a localized flaky unit-test timeout fix.

Changes

• Increase the watch producer span-release test timeout from 200 ms to five seconds.
• Poll the exporter every 10 ms, consistent with the neighboring asynchronous test pattern.
• Audit the remaining sub-second server test timeouts; they are intentional negative assertions or per-attempt TLS handshakes within a longer deadline, so no other tests need this change.

Testing

• mise run pre-commit passes
• mise run test:rust passes
• Focused watch producer span-release test passes
• E2E tests added/updated (not applicable: test-only timeout change)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

[rhuss]

This test is still flaking on ARM64 after the fix. Hit it in #2600 (docs-only change):

assertion `left == right` failed: watch producer should release the request span after client disconnect
  left: 0
 right: 1

Rust (linux-arm64-cpu8) job, sandbox.rs:2659. Passes on x86. The stabilization may not account for ARM's different scheduling/timing behavior.