Prevent generated RPC methods from being shadowed by ServiceStub parameters

[gaoflow]

Fixes #224.

Problem

Generated RPC methods are class-level methods on the ServiceStub subclass. ServiceStub.__init__ stored its channel/timeout/deadline/metadata parameters as plain instance attributes. When a generated RPC's snake_cased name collides with one of those names — e.g. an RPC literally named Metadata becomes metadata — the instance attribute shadows the class method, since instance attributes take precedence over class methods during attribute lookup. Calling the RPC then fails with TypeError: 'dict' object is not callable (or 'NoneType' object is not callable when no metadata was passed).

Fix

Store the four constructor parameters under private names (self._channel, etc.) and expose them through read-only @property accessors. A subclass method now overrides the base property in the MRO, so a colliding RPC method is reachable again. Non-colliding access (stub.channel, stub.metadata, …) still returns the parameter value exactly as before, so this is backwards compatible for normal reads. The properties are read-only; the parameters were never reassigned after __init__, so nothing in the codebase relied on writing to them.

Tests

Added tests/grpc/test_service_stub_attributes.py:

• test_rpc_method_not_shadowed_by_constructor_param — a ServiceStub subclass defining an async metadata method (the colliding RPC); asserts the method is still callable and returns its own result. Fails before the change (metadata resolves to the dict), passes after.
• test_constructor_params_exposed_as_properties — asserts channel/timeout/deadline/metadata properties return the values passed to __init__.