Add per-test execution cancellation

[thomhurst]

Summary

• add TestContext.Execution.Cancel() for cooperative, test-scoped cancellation
• create one lightweight cancellation source per executed attempt and propagate it through injected tokens, timeouts, and custom test executors
• mark the execution Cancelled even when the body or a custom executor consumes the cancellation exception
• ignore late cancellation requests after execution completes
• document usage and update public API snapshots

Performance

A .NET 10 microprobe measured source creation and disposal at approximately:

• 48 B and 11 ns with a non-cancellable parent token
• 64 B and 47 ns with a cancellable parent token

There is no background task, polling, TaskCompletionSource, or Task.WhenAny overhead.

Validation

• dotnet build TUnit.Dev.slnx --no-restore -graphBuild:True
• focused cancellation/custom executor/timeout integration tests
• existing linked cancellation regression tests
• public API verification on net472, net8.0, net9.0, and net10.0
• documentation production build

yarn typecheck remains blocked by the existing TypeScript 7 baseUrl incompatibility in docs/tsconfig.json; the Docusaurus production build succeeds.

Stack

Based on #6568 because its linked cancellation ownership changes are the prerequisite for composing this API with AddLinkedCancellationToken and [Timeout]. Retarget to main after #6568 merges.

[greptile-apps]

Greptile Summary

The PR adds cooperative, per-test execution cancellation and integrates it with retry transitions, timeout tokens, custom executors, documentation, and public API snapshots.

• Adds ITestExecution.Cancel() and per-attempt cancellation-source lifecycle management.
• Preserves accepted cancellation across retries and converts consumed cancellation into a cancelled terminal result.
• Adds integration and unit coverage for cancellation, retry backoff, transition races, and late calls.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/TUnit.Core/TestContext.Execution.cs	Adds the public cancellation implementation, synchronization, retry preservation, and deferred source disposal; the previously reported races are addressed at current HEAD.
src/TUnit.Engine/TestExecutor.cs	Routes test execution and timeout handling through the per-test token and preserves cancellation during retryable attempt transitions.
src/TUnit.Engine/Services/TestExecution/RetryHelper.cs	Keeps cancellation active through retry decisions and backoff and preserves cancelled results when retry callbacks throw.
tests/TUnit.Engine.Tests/TestExecutionCancellationTests.cs	Adds end-to-end assertions for normal cancellation, late calls, retry backoff, retry transitions, and retry callback exceptions.
tests/TUnit.UnitTests/TestContextCancellationTests.cs	Adds focused synchronization tests covering deferred disposal, source replacement, and cancellation between attempts.
docs/docs/execution/cancellation.md	Documents cooperative test-scoped cancellation, callback usage, lifecycle behavior, and composition with existing timeout and token APIs.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Test attempt begins] --> B[Initialize per-attempt cancellation source]
  B --> C[Test body / custom executor]
  C --> D{Cancel requested?}
  D -->|Yes| E[Signal injected and execution tokens]
  D -->|No| F{Attempt failed and retryable?}
  E --> G[Record Cancelled]
  F -->|Yes| H[Keep cancellation open through retry decision and backoff]
  H --> I[Initialize next attempt while preserving request]
  I --> C
  F -->|No| J[Complete cancellation acceptance]
  J --> K[Finalize result and dispose sources]
  G --> K

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_{Reviews (8): Last reviewed commit: "fix(engine): close retry cancellation ga..." | Re-trigger Greptile}