GenFacades: Enrich PNSE stubs with XML doc comments from implementation sources

[Copilot]

NotSupportedAssemblyGenerator generated PNSE stubs with no useful XML docs — the shipped PNSE assemblies lacked the documentation that matched the implementation. This adds a doc-comment enrichment pass: all **/*.cs files in the project are parsed for /// docs and applied to the generated stubs.

Approach

Rather than complex cross-project source evaluation, the task does a pre-pass over @(NotSupportedAssemblyImplementationSource) (defaults to **/*.cs in the current project) to build a DocCommentIndex (API key → trivia, first-occurrence-wins), then the rewriter applies those docs when generating each stub. API definitions still come from the ContractProject reference sources; the implementation sources only contribute documentation.

Key changes

NotSupportedAssemblyGenerator.cs

• New ImplementationSourceFiles task property
• DocCommentIndex — lookup from FQN API key → SyntaxTriviaList of doc-comment trivia; first match across all impl files wins
• DocCommentWalker (CSharpSyntaxWalker) — populates the index across all member kinds: types, methods, properties, events (including event-field syntax), constructors, fields, operators, conversion operators, enum members
• ApiKey static class — shared FQN key computation used by both walker and rewriter; handles regular and file-scoped namespaces (C# 10), nested types, and top-level types
• ReplaceDocTrivia — strips existing doc-comment trivia from the reference node's leading trivia, inserts impl docs before the final indentation whitespace
• ApplyDocComment — called from every Visit* method; emits LogWarning for APIs not found in impl or found without docs (only when impl sources are provided)
• Extracted CreateThrowBlock() / CreateEmptyBlock() to eliminate the repeated ParseStatement pattern

Microsoft.DotNet.GenFacades.NotSupported.targets

• Adds NotSupportedAssemblyImplementationSource item (defaults to **/*.cs, excludes $(IntermediateOutputPath))
• Opt-out: set IncludeNotSupportedAssemblyImplementationSources=false or remove items from NotSupportedAssemblyImplementationSource
• Removed ImplementationProject approach from previous iteration

To double check:

• The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md