feat(mediator): add ExceptionHandlingBehavior pipeline

[senrecep]

Summary

• Adds ExceptionHandlingBehavior<TRequest, TResponse> — catches unhandled handler exceptions and converts them to Result.Failure(Error.Exception(ex))
• Registered as singleton between LoggingBehavior and CachingBehavior so callers always receive a typed failure instead of a raw exception
• Zero overhead when TResponse is not Result/Result<T> (fast path skips wrapping entirely)
• OperationCanceledException always propagates — never caught
• Adds AddMediatorExceptionHandlingBehavior() selective registration method
• 10 new tests (all passing); SKILL.md and API_REFERENCE.md updated with working code examples

Pipeline order (after this PR)

Position	Behavior	Lifetime
1	ValidationBehavior	Scoped
2	LoggingBehavior	Singleton
3	ExceptionHandlingBehavior ← new	Singleton
4	CachingBehavior	Singleton
5	TransactionScopeBehavior	Singleton

Test plan

• dotnet build — 0 errors, 0 warnings (all TFMs)
• dotnet test --filter "FullyQualifiedName~Mediator" — 33/33 passed
• Happy path (Result, Result) passes through unchanged
• Exception → Result.Failure(Error.Exception(ex)) with correct code/description
• OperationCanceledException propagates
• Non-Result TResponse → exception propagates unchanged
• Async handler throw (after await) caught correctly

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ls.Mediator/Behaviors/ExceptionHandlingBehavior.cs	91.42%	2 Missing and 1 partial ⚠️
...Mediator/DependencyInjection/MediatorExtensions.cs	25.00%	3 Missing ⚠️

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