csharp: implement TreeVisitor.Adapt() on the .NET side

[jkschneider]

Summary

• Mirrors the Python fix from c89d246 (python: implement TreeVisitor.adapt() so generic visitors can visit Py/J nodes #7590) for the parallel .NET implementation in rewrite-csharp/csharp/OpenRewrite/. Two distinct failure modes existed before this change:

1. Silent no-op: A bare TreeVisitor<J, P> (or any subclass that overrides only PreVisit / PostVisit — a common pattern for cross-language collectors) traversing a parsed C# source would silently no-op past the root. The default Accept is identity, so traversal never descended into children. Quiet, but every per-node hook missed every node.

2. Hard throw: A plain JavaVisitor<P> traversing a Cs source would throw InvalidOperationException("Unknown J tree type: ...") on the first Cs.UsingDirective / Cs.PropertyDeclaration / etc. its switch doesn't recognize.

Unlike the JVM side (which inherits a real TreeVisitor.adapt() from rewrite-core backed by Quarkus Gizmo bytecode generation) and unlike the TypeScript side (which sidesteps the problem with a kind-keyed registry consulted only inside JavaVisitor.accept), the C# port had no adaptation mechanism at all.

Approach

• TreeVisitorAdapterRegistry — a non-generic, lock-protected registry of (treeType, openLangVisitorType, openAdapterType) triples kept sorted with the most specific tree type first, so a Cs node hits the CSharpVisitor adapter before falling through to the JavaVisitor one.

• TreeVisitor<T, P>.Adapt(tree) — looks up the matching language adapter, closes the open generic with P, and instantiates via Activator. Result is cached per visitor instance keyed by the closed language-visitor type, so the wrapper allocation happens at most once per language per visitor lifetime.

• Visit() now dispatches via Adapt(t).Accept(t, p) instead of Accept(t, p). Cursor is made virtual so the adapter can forward it to the wrapped visitor.

• TreeVisitorAsJavaVisitor<P> / TreeVisitorAsCSharpVisitor<P> — thin adapter classes that IS-A JavaVisitor / CSharpVisitor (so the language switch and child-traversal defaults are inherited), but override Visit to forward to the wrapped visitor (so its lifecycle — _visitCount, _afterVisit, _stopAfterPreVisit, PreVisit, PostVisit, DefaultValue — runs unchanged) and override Cursor to forward to wrapped state. Each language registers its triple via a [ModuleInitializer] that fires on assembly load, so registration is guaranteed before any TreeVisitor<J, P>.Visit() call regardless of which generic instantiations the user code happens to reference.

Test plan

• New TreeVisitorAdaptTest covers both regression modes:
  • BareTreeVisitorTraversesCSharpSource: a TreeVisitor<J, ExecutionContext> overriding only PreVisit descends into both Cs- and J-typed nodes of a parsed file.
  • JavaVisitorTraversesCSharpSourceWithoutThrowing: a JavaVisitor<ExecutionContext> traverses the same source without throwing on Cs nodes.
• Full C# suite green: 1917 / 1917 passing locally with RPC_TEST_SERVER_CLASSPATH wired via ./gradlew :rewrite-csharp:rpcTestClasspath (covers JVM↔.NET RPC integration tests too).
• No changes to public API surface beyond making Cursor virtual and adding Adapt (protected virtual). Existing JavaVisitor / CSharpVisitor / XmlVisitor overrides of Accept continue to compile unchanged.