Fix #4715: Propagate CancellationToken through property injection to prevent hangs

[thomhurst]

Summary

• Threads the 5-minute discovery timeout CancellationToken from TestDiscoveryService.DiscoverTestsStreamAsync through the entire property injection chain: TestBuilderPipeline → TestBuilder → ObjectLifecycleService → PropertyInjector → EnsureInitializedAsync
• When IAsyncInitializer.InitializeAsync() hangs during property injection in a chained ClassDataSource dependency pattern, the timeout now fires and unblocks all waiters via OperationCanceledException
• Adds reproduction test cases covering: immediate failures, slow init (30s delay), discovery-phase failures, attribute-based injection, background process scenarios, and shared object multi-test failures

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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This PR implements a clean solution to the hang issue by threading the cancellation token through the entire property injection chain. The changes are:

• Mechanically correct: All method signatures are updated consistently, with CancellationToken added as the last optional parameter with default values
• CLAUDE.md compliant: Works for both source-gen and reflection modes (shared runtime logic), no public API changes, minimal performance impact
• Well-tested: Comprehensive test coverage including immediate failures, slow initialization, attribute-based injection, and background process scenarios

The architecture properly leverages .NET's cancellation infrastructure to unblock waiting tests when initialization fails or times out, converting an indefinite hang into a graceful failure with OperationCanceledException.