fix: fix function object returned from RPC to be a callable

[ryanking13]

This fixes some bugs related to RPC calls, that I noticed while checking RPC docs examples.

1. When RPC call returns a function (closure), it was not interpreted as callable.
2. calling bool(binding) was failing because the Python class fallbacks to len() call when there is no __bool__ implemented.

[ask-bonk]

The code looks solid. Let me verify one more thing about the __len__ change — whether float values from JS could sneak through (JS numbers are always floats, but Pyodide may convert integer-valued floats to Python int):

The isinstance(length, int) check is correct because:

• If self._binding is an RPC stub, getattr(self._binding, "length", None) will return a JsProxy method/property (not an int), so the check correctly rejects it.
• If it's a real array-like with .length, Pyodide converts integer JS numbers to Python int, so the check passes.

The tests are well-structured and cover:

• Callable RPC stubs (both Python-to-Python and Python-to-JS)
• bool() on bindings and stubs
• Chained D1 statements staying as _BindingWrapper (not _RpcStubWrapper)
• len() still raising TypeError on objects without length
• list() raising TypeError on non-iterable stubs

The test files import _BindingWrapper and _RpcStubWrapper directly from workers.rpc, which are private names (prefixed with _). This is fine for internal tests.

Everything here looks correct and well-tested. The changes are minimal and focused on the two bugs described in the PR.

LGTM

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