refactor: apply Interface Segregation to TUnitServiceProvider

[thomhurst]

Summary

Addresses #4898 - TUnitServiceProvider exposes 35+ public properties, violating the Interface Segregation Principle.

• Extracts three focused interfaces from TUnitServiceProvider so consumers depend only on the services they actually need:
  • ITestDiscoveryServices: DiscoveryService, TestBuilderPipeline, TestFilterService, TestFinder
  • ITestExecutionServices: TestExecutor, TestSessionCoordinator, CancellationToken, FailFastCancellationSource, AfterSessionHooksFailed
  • ILoggingServices: Logger, VerbosityService, MessageBus
• Updates TestSessionCoordinator to depend on ITestExecutionServices instead of the full TUnitServiceProvider
• Updates IRequestHandler and TestRequestHandler to accept the specific interfaces they need
• TUnitServiceProvider implements all three interfaces, so TUnitTestFramework can pass it directly

This is an incremental first step -- further consumers and properties can be migrated to narrower interfaces in follow-up work.

Test plan

• dotnet build TUnit.Engine/TUnit.Engine.csproj passes with zero errors and zero warnings
• Existing tests continue to pass (no behavioral changes, pure refactoring)

[claude]

Code review

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

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This is a clean, well-executed Interface Segregation refactor. A few notes worth calling out:

What's done well:

• All three new interfaces (ILoggingServices, ITestDiscoveryServices, ITestExecutionServices) are properly scoped as internal, so no public API surface is changed and no snapshot test updates are needed.
• The call site in TUnitTestFramework correctly passes serviceProvider three times (once per interface). Since TUnitServiceProvider implements all three, the compiler resolves each argument to the correct interface type — no runtime overhead or object allocation introduced.
• The circular dependency in TUnitServiceProvider's constructor (executionServices: this before TestSessionCoordinator is assigned) is safe: TestSessionCoordinator only stores the reference and doesn't dereference executionServices.TestSessionCoordinator until after full construction.
• TestSessionCoordinator correctly accesses only the three members it needs (FailFastCancellationSource, CancellationToken, AfterSessionHooksFailed) — all of which are present on ITestExecutionServices.
• CLAUDE.md compliance: Dual-Mode rule is not triggered (this is post-metadata-collection orchestration only), AOT compatibility is preserved, no async blocking introduced.

One design note (not a blocker): AfterSessionHooksFailed { get; set; } on ITestExecutionServices exposes mutable shared state through an interface, which is a design smell. It was already present on TUnitServiceProvider before this PR, so this isn't a regression — but it's worth addressing in a follow-up alongside the incremental migrations mentioned in the PR description.

Overall, this is a solid first step in the incremental ISP migration.