fix: detect Context parameter in callable class instances

[giulio-leone]

Summary

Fix find_context_parameter() to correctly detect the Context parameter in callable class instances (objects with __call__ method).

Problem

When registering a callable class instance as an MCP tool via add_tool(), the ctx: Context parameter is incorrectly exposed as a visible tool parameter in the JSON schema instead of being injected by the framework.

This happens because typing.get_type_hints() raises TypeError on callable class instances ('<MyTool object>' is not a module, class, method, or function). The exception is caught, find_context_parameter() returns None, and the context parameter is not excluded from the schema.

Solution

Before calling get_type_hints(), detect callable class instances and inspect their __call__ method instead:

target = fn
if not (inspect.isfunction(fn) or inspect.ismethod(fn)):
    if callable(fn) and hasattr(fn, '__call__'):
        target = fn.__call__

This is consistent with how _is_async_callable() (in tools/base.py) already handles callable classes.

Test

Added test_context_injection_callable_class that registers a callable class with a Context parameter and verifies it is properly injected (not exposed in schema).

Fixes #1974