Add an ExportedTypes RPC to enumerate a package's public API

[jkschneider]

What

Adds an ExportedTypes RPC method to each language engine (C#, JavaScript, Go, Python) that enumerates the public types a set of own artifacts (assemblies / package directories / module directories) export into OpenRewrite JavaType.

• Public types the own artifacts define come back with complete bodies — members, methods, supertype, interfaces.
• Types they only reference come back shallow (FQN only) for the caller to resolve.

This lets a caller read a dependency's public API without re-parsing its sources.

Artifact-scoped resolution

The enumeration is a function of the artifact alone — the referenced packages are never enumerated:

• C# reads the .dll's TypeRef FQNs directly, so a reference into an assembly that wasn't loaded still comes back as a shallow FQN class-ref rather than Unknown (only the BCL is supplied, so primitives resolve).
• Go resolves the module's own go.mod requires (in the module cache) to name cross-module references.
• Python derives the ty environment from the artifact and resolves against the package's own declared dependencies.

How

The response streams and paginates:

• The engine sends the FQNs the artifacts define first, so the caller can tell defined types from references before the first type arrives, then the types as a ref-deduplicated object stream (the same wire format getObject uses).
• RpcReceiveQueue gains a streaming receiveList(before, onChange, sink) overload so a client never materializes the whole list.
• Each response is returned in fixed-size (1000-item) slices across the client's repeated identical requests, so no single message carries a large result.

Per-module

Module	Enumerator	RPC
rewrite-core	—	streaming RpcReceiveQueue.receiveList overload
rewrite-csharp	AssemblyTypeEnumerator (Roslyn)	ExportedTypes handler + CSharpRewriteRpc.exportedTypes
rewrite-javascript	exportedTypes (TypeScript checker)	handler + JavaScriptRewriteRpc.exportedTypes
rewrite-go	ExportedTypes (go/types)	handler + GoRewriteRpc.exportedTypes
rewrite-python	handler (ty)	PythonRewriteRpc.exportedTypes

Tests

• Enumerator tests per engine (own types complete with methods; references shallow; module-boundary and per-OS/alias correctness for Go).
• Handler pagination tests per engine (slices reassemble the single-shot response exactly; cache evicted after drain).
• C# integration test round-trips Newtonsoft.Json over the wire.