[Sonic] Test cleanup

[davidwengier]

Completion was the last feature that needed work, but it seems there were some tests still outstanding.

This is essentially the last tooling PR for sonic, other than removing obsolete attributes etc. once we decide on API shape etc. As far as tooling is concerned though, I believe this means the feature branch can be merged.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This change set updates Razor/Sonic tooling test infrastructure and unit tests to account for the generated C# impl/decl split, and re-enables previously skipped rename/code-action test coverage by mapping edits across both generated documents when present.

Changes:

• Re-enabled previously skipped cohost rename/code-action tests and updated them to choose the correct generated document (impl vs decl) based on document mapping.
• Updated span/text-change mapping in tests to consider both generated documents (impl + optional decl) when validating Razor edit remapping.
• Refactored/updated supporting test infrastructure to use the newer GetRequiredCSharpDocument(declarationDocument: false) API and to track optional declaration output where applicable.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Razor/src/Razor/test/Microsoft.VisualStudioCode.RazorExtension.UnitTests/Endpoints/Rename/RoslynRenameTest.cs	Re-enables the VS Code cohost rename test.
src/Razor/src/Razor/test/Microsoft.VisualStudio.LanguageServices.Razor.UnitTests/Cohost/CohostRoslynRenameTest.cs	Updates rename tests to select impl/decl generated docs correctly and map changes across both.
src/Razor/src/Razor/test/Microsoft.VisualStudio.LanguageServices.Razor.UnitTests/Cohost/CohostRoslynCodeActionTest.cs	Updates code action tests to map edits against both generated docs and factors mapping into a helper.
src/Razor/src/Razor/test/Microsoft.CodeAnalysis.Razor.CohostingShared.UnitTests/RazorDocumentMappingServiceTest.cs	Migrates mapping tests to use GetRequiredCSharpDocument(declarationDocument: false).
src/Razor/src/Razor/test/Microsoft.CodeAnalysis.Razor.CohostingShared.UnitTests/Microsoft.CodeAnalysis.Razor.CohostingShared.UnitTests.projitems	Removes the deleted excerpt test base from compilation inputs.
src/Razor/src/Razor/test/Microsoft.CodeAnalysis.Razor.CohostingShared.UnitTests/DocumentExcerpt/RazorDocumentExcerptServiceTest.cs	Inlines former base-class setup and adds coverage for declaration-document excerpt/classification scenarios.
src/Razor/src/Razor/test/Microsoft.AspNetCore.Razor.Test.Common.Tooling/Language/IntegrationTests/RazorToolingIntegrationTestBase.cs	Updates integration test compilation pipeline to handle optional decl output and adjusts syntax tree tracking accordingly.