Replace ETW events with incremental testing framework in RazorSourceGeneratorTests

[Copilot]

Summary of the changes

RazorSourceGeneratorTests relied on ETW events to verify generator execution, which forced sequential test execution due to global event contamination. Migrated to Roslyn's incremental generator testing framework.

Generator changes:

• Added .WithTrackingName() to 8 pipeline steps (RazorSourceGeneratorOptions, GeneratedDeclarationCode, TagHelpersFromCompilation, TagHelpersFromReferences, ParsedDocuments, RewrittenTagHelpers, CheckedAndRewrittenTagHelpers, GeneratedCode)

Test infrastructure:

• Added VerifyIncrementalSteps() and VerifyIncrementalStepsMultiple() helpers to verify step execution via TrackedSteps API
• Removed RazorEventListener class and all ETW assertions (~290 lines)
• Removed [Collection] attribute enabling parallel test execution

Test updates:

// Before: ETW events
Assert.Collection(eventListener.Events,
    e => Assert.Equal("ParseRazorDocumentStart", e.EventName),
    e => Assert.Equal("ParseRazorDocumentStop", e.EventName));

// After: Incremental framework
result.VerifyIncrementalSteps("ParsedDocuments", 
    IncrementalStepRunReason.New, IncrementalStepRunReason.New);

Tests now track execution reasons (New, Cached, Modified, Unchanged) instead of just event occurrence, providing better granularity and enabling parallel execution.

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Original prompt

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This section details on the original issue you should resolve

<issue_title>RazorSourceGeneratorTests should use incremental framework instead of ETW</issue_title>
<issue_description>Today the tests in RazorSourceGeneratorTests use the ETW events to determine which parts of the generators ran. This is fragile as parallel tests can contribute to this, meaning we have to run them in sequential order.

Instead, we should use the incremental testing framework, an example of which is here: https://github.com/dotnet/runtime/blob/d78c24c497f5ae9046d6f61f85d0066d8a159b60/src/libraries/Microsoft.Extensions.Logging.Abstractions/tests/Microsoft.Extensions.Logging.Generators.Tests/LoggerMessageGeneratorIncrementalTests.cs

We already partially do this, but should completely replace all uses of ETW with it. We may need to add more .WithTrackingName calls to the generator itself in order to access each step that is currently tracked with ETW.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes RazorSourceGeneratorTests should use incremental framework instead of ETW #12759

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