Fix failover CI by bypassing mock auth in failover tests

[Copilot]

The failover Actions job started failing because the shared livekit/test-server now enforces auth by default, causing failover tests to fail with 401 unauthenticated before retry logic is exercised. This change updates the failover test harness to explicitly opt out of auth for these scenarios.

• Problem context

  • Failover tests are intended to validate retry/region behavior, not permission enforcement.
  • Placeholder test token now trips server-side auth checks and blocks those paths.

• Change

  • In tests/api/test_failover.py, update request headers in _call(...) to always include x-lk-mock-skip-auth: true.
  • Keep existing per-test X-Lk-Mock-* directives unchanged and composable.

• Effect on test behavior

  • Requests reach failover code paths again (healthy, 5xx retry, transport-drop retry, terminal 4xx, discovery failure) instead of terminating on auth.

headers = {
    "authorization": "******",
    "x-lk-mock-skip-auth": "true",
    **directives,
}

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.