Make initial route timeout tests owner-driven

[lawrencecchen]

Tests for initial route fallback used the same 20 ms wall-clock deadline for the hanging route and the valid Unix fallback. Under Valgrind, the fallback also expired.

This change injects a test-only deadline owner. Each test waits for the hanging provider or link to start, expires only that route, and awaits Unix selection under one final watchdog. Production still uses one absolute Tokio deadline across provider and link setup.

Red evidence: https://github.com/manaflow-ai/cmux/actions/runs/31498625744

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Summary by cubic

Makes initial-route timeout tests owner-driven to fix Valgrind flakes. Tests now expire only the hanging route and reliably fall back to Unix, while production behavior stays the same.

• Bug Fixes
  • Added InitialAttemptDeadlineOwner with a manual test mode to control per-route timeouts.
  • Introduced connect_first_available_with_deadline_owner and used it in tests.
  • Updated tests to wait for the hanging provider/link to start, then expire that route and select the Unix fallback; production still uses a single absolute tokio deadline.

^{Written for commit 9bce672. Summary will update on new commits.}

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