Register JavaType.ShallowClass codec on the Python RPC side

[knutwannheden]

Motivation

Production SB4 runs in rewrite-pip (the Python-side RPC runtime) were failing with:

io.moderne.jsonrpc.JsonRpcException: {code=-32603, message='No RPC codec registered on
the Python side for 'org.openrewrite.java.tree.JavaType$ShallowClass'. The remote side
has a codec and sent property messages that will not be consumed, causing RPC queue
desynchronization.'}

JavaType.ShallowClass is the placeholder used when full JVM type resolution isn't
available. It's a subclass of JavaType.Class with the same wire shape but a distinct
type name — and the Python RPC side wasn't registering a codec for it, so any Java
recipe that sent a ShallowClass to the Python process deadlocked the queue.

I audited the three non-Java RPC implementations:

• Python (rewrite-python) — no codec registered for JavaType$ShallowClass.
  Fixed here.
• C# (rewrite-csharp) — already works. FromJavaTypeNameByConvention resolves
  org.openrewrite.java.tree.JavaType$ShallowClass to the existing nested
  OpenRewrite.Java.JavaType+ShallowClass class via reflection, and
  JavaReceiver.VisitType's case JavaType.Class cls: matches ShallowClass through
  inheritance. On the send side, ToJavaTypeName handles nested types recursively and
  produces the correct Java FQN.
• Go (rewrite-go) — already works for the receive path. The factory for
  JavaTypeShallowClassKind is registered in rewrite-go/pkg/rpc/registry.go,
  producing a *JavaTypeClass instance that the visitor dispatches to VisitClass.
  Go has no separate JavaTypeShallowClass struct by design — all class-like types
  collapse to *JavaTypeClass, so the Go process never constructs a ShallowClass
  to send.

Summary

• Factored out _receive_java_type_class_fields in python_receiver.py so the same
  field-consumption logic can populate either a JT.Class or a JT.ShallowClass.
• Registered a codec + factory for org.openrewrite.java.tree.JavaType$ShallowClass
  on the Python side.
• On the sender side, added an isinstance(obj, JavaType.ShallowClass) check before
  the JavaType.Class check in send_queue.py so ShallowClass instances round-trip
  with the correct Java type name (the existing check would have collapsed them into
  JavaType$Class because ShallowClass inherits from Class).

Test plan

• Verified the codec and factory are registered under the expected Java type
  name, and that RpcSendQueue._get_value_type() returns
  JavaType$ShallowClass for a JT.ShallowClass() and JavaType$Class for a
  JT.Class().

This PR does not by itself fix the production failure — the fix lands on the
Python-side runtime and needs a rewrite-pip release before the next SB4 flagship
run will pick it up.