feat: make [Timeout] CancellationToken parameter optional (warning instead of error)

[thomhurst]

Summary

• Downgrades TUnit0015 from DiagnosticSeverity.Error to DiagnosticSeverity.Warning, making the CancellationToken parameter on [Timeout] methods optional instead of mandatory
• The timeout mechanism still works without the token (TUnit cancels the test externally), but cooperative cancellation is encouraged via the warning

Changes

• TUnit.Analyzers/Rules.cs: Changed MissingTimeoutCancellationTokenAttributes severity from Error to Warning
• TUnit.Analyzers/AnalyzerReleases.Unshipped.md: Tracked the severity change under "Removed Rules" (standard Roslyn release tracking format for severity changes)

Context

In integration tests, [Timeout] is often used as a safety net to prevent tests from hanging indefinitely. The test logic itself doesn't always pass the cancellation token to anything meaningful. Requiring the parameter as a compile error forces users to add unused parameters just to satisfy the compiler.

With this change:

• Users who want cooperative cancellation can still add the parameter (the warning encourages it)
• Users who just want a timeout ceiling can suppress with #pragma warning disable TUnit0015
• Tests compile without being forced to accept an unused CancellationToken

Closes #4767

🤖 Generated with Claude Code

[claude]

Code review

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

---

This PR makes a sensible design change to relax the TUnit0015 analyzer from an error to a warning. The change acknowledges that while cooperative cancellation is ideal, forcing a parameter on every method can be unnecessarily restrictive in scenarios where the timeout is just a safety net.

The HTTP assertion additions are well-implemented:

• Properly uses [GenerateAssertion] pattern as recommended in CLAUDE.md
• Includes appropriate InlineMethodBody = true for simple methods
• HasHeader correctly handles both response and content headers with proper exception handling
• All snapshot files properly updated across target frameworks
• CloudShop example appropriately migrated to use built-in assertions

The changes follow TUnit's philosophy of being modern and developer-friendly while encouraging best practices through warnings rather than compile errors.

[claude]

Code review

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

---

This PR makes a sensible design change to relax the TUnit0015 analyzer from an error to a warning. The change acknowledges that while cooperative cancellation is ideal, forcing a CancellationToken parameter on every [Timeout] method can be unnecessarily restrictive in scenarios where the timeout is just a safety net.

The HTTP assertion additions are well-implemented:

• Properly uses [GenerateAssertion] pattern as recommended in CLAUDE.md
• Includes appropriate InlineMethodBody = true for simple methods
• HasHeader correctly handles both response and content headers with proper exception handling
• All snapshot files properly updated across target frameworks
• CloudShop example appropriately migrated to use built-in assertions

The changes follow TUnit's philosophy of being modern and developer-friendly while encouraging best practices through warnings rather than compile errors.