JNI: fix non-compiling async thunks for protocol boxes and generic types

[Flatout73]

Motivation

Async methods in JNI mode capture self into the Task via nonisolated(unsafe)
bindings. The capture always referenced <param>$, but that binding only exists for
class/actor lowering (.extractSwiftValue). Protocol boxes load self into
<param>Existential$, and generic-Self downcalls render inside the opener extension
where selfTypePointer is never bound — so the generated Swift referenced unbound
identifiers and failed to compile.

Changes

• Capture the binding the self lowering actually produced: <param>$ for
  classes/actors, <param>Existential$ for protocol boxes.
• Stop capturing the type-metadata parameter; the opener thunk neither receives
  nor binds it, and the task body never uses it.

Testing

New JNIAsyncSelfCaptureTests pin the generated captures for all three self
shapes (class, protocol box, generic class); the generic and protocol tests fail
without this change. Full test suite passes (654 tests).

🤖 Fixed with help of Claude Code

#865