Remove JavaMethodWrapper and decouple JavaModuleWrapper from legacy invocation (facebook#56201)

Summary:
Pull Request resolved: facebook#56201

JavaMethodWrapper is a legacy architecture class that wraps ReactMethod-annotated
Java methods via reflection for the bridge-based NativeModule invocation path.

When ReactNativeFeatureFlags.useTurboModuleInterop is enabled (the new architecture),
this class is completely bypassed — the TurboModule interop layer uses
JavaInteropTurboModule (C++) with direct JNI invocation via
JavaTurboModule::invokeJavaMethod(), which converts JSI values to JNI arguments
directly in C++ without any Java-side reflection.

This diff:
- Deletes JavaMethodWrapper.kt (only instantiated from JavaModuleWrapper.findMethods())
- Deletes BaseJavaModuleTest.kt (test that exercised JavaMethodWrapper)
- Removes the NativeMethod interface from JavaModuleWrapper (only implemented by JavaMethodWrapper)
- Rewrites JavaModuleWrapper.findMethods() to compute method type inline without JavaMethodWrapper
- Changes JavaModuleWrapper.invoke() to throw UnsupportedOperationException
- Sets md.signature to empty string for sync methods to prevent C++ null deref

The JavaModuleWrapper class shell is kept because it is still referenced from C++ JNI
(JavaModuleWrapper.cpp) and NativeModuleRegistry. Full removal is planned as follow-up.

Changelog: [Internal] internal

Reviewed By: cortinico, javache

Differential Revision: D97387121

fbshipit-source-id: 5c6a16f5b8d92430e4afeb3603b5bc538c076f90

Author: mdvacca