Bound Claude OAuth test gates

[cbusillo]

Summary

• replace Claude OAuth polling helpers with a bounded continuation-backed start gate
• keep release gates explicit so real locks and token POSTs cannot self-open
• guarantee cleanup and preserve the original timeout reason through teardown cancellation
• add a direct start-timeout regression test

Refs #578

Validation

• four focused Claude OAuth gate tests passed
• 25 repeated focused stability runs passed
• lock-contention bypass mutation failed on expected error, HTTP request, and credential-save assertions
• routine Swift lane passed 892 tests
• scripts/commit-gate.sh
• JetBrains changed-files inspection: GREEN, 0 findings
• Opus and Gemini exact-SHA final reviews approved 5da35f2

Scope

• test-only change in Tests/ContextPanelCoreTests/ProviderConnectorTests.swift
• no production lock or OAuth behavior changed