Fix Roslyn analyzer tutorial: Correct test case categorization

[Copilot]

The "Create tests for valid declarations" section incorrectly categorized some test cases as failing when they actually pass due to existing analyzer logic.

Problem

The tutorial stated that four test cases "you haven't handled yet" would fail when running tests:

• VariableIsAlreadyConst_NoDiagnostic
• NoInitializer_NoDiagnostic
• InitializerIsNotConstant_NoDiagnostic
• MultipleInitializers_NoDiagnostic

However, the first two tests actually pass because the analyzer already handles these conditions:

• Already const declarations: Filtered out by the const keyword check in the analyzer
• No initializer declarations: Handled by data flow analysis that detects assignments outside the declaration

Solution

Reorganized the documentation to correctly categorize the tests:

Tests that pass (already handled):

• Variable assigned after initialization (existing example)
• Variable already declared as const (moved from "failing" section)
• Variable with no initializer (moved from "failing" section)

Tests that actually fail (need to be implemented):

• Variable initializer is not a constant
• Multiple variables where not all can be made constant

Updated the text from "you'll see these new test cases fail" to "you'll see these last two test cases fail" for accuracy.

Impact

Tutorial readers will no longer be confused by tests that the documentation claims should fail but actually pass, making the learning experience clearer and more accurate.

Fixes #23501.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/roslyn-sdk/tutorials/how-to-write-csharp-analyzer-code-fix.md	docs/csharp/roslyn-sdk/tutorials/how-to-write-csharp-analyzer-code-fix

[BillWagner]

Overall, this is great, but I had one additional task before we say this is ready.

[BillWagner]

Thanks @copilot

This LGTM.