feat: add TestFixAll method to CodeFixTestFixture

[Arthurvdv]

Summary

Adds FixAll testing support to CodeFixTestFixture to catch bugs where individual code fixes work but applying all fixes simultaneously fails (e.g., overlapping text changes silently dropped by BatchFixer).

New Public API

// Primary: markup with [| |] markers + expected output + diagnostic ID
fixture.TestFixAll(markupCode, expectedCode, diagnosticId);

// With DiagnosticDescriptor
fixture.TestFixAll(markupCode, expectedCode, DiagnosticDescriptors.Rule);

How it works

1. Parses [| |] markers to identify expected diagnostic locations
2. Runs the analyzer, asserts diagnostic count matches marker count
3. Auto-detects EquivalenceKey from the first diagnostic's CodeAction
4. Creates a FixAllContext with a custom DiagnosticProvider at FixAllScope.Document scope
5. Invokes the CodeFix's real FixAllProvider.GetFixAsync()
6. Compares the result against the expected code string

Files changed

File	Change
TestDiagnosticProvider.cs	New: implements FixAllContext.DiagnosticProvider
CodeFixTestFixture.cs	Add TestFixAll overloads
RoslynTestKitException.cs	Add FixAll-specific error factories
README.md	Add TestFixAll examples and update folder structure