fix: detect Context parameter in callable class instances

[giulio-leone]

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 find_context_parameter() uses typing.get_type_hints(fn), which doesn't introspect the __call__ method of callable class instances.

class MyTool:
    def __init__(self, name: str):
        self.__name__ = name

    async def __call__(self, query: str, ctx: Context) -> str:
        return f"Result: {query}"

mcp.add_tool(MyTool(name="my_tool"), name="my_tool")
# Bug: 'ctx' appears as a required parameter in the schema

Fix

In find_context_parameter(), detect callable class instances (not functions or methods) and inspect fn.__call__ instead of fn directly.

Tests

Added two tests:

• test_context_detection_callable_class: Verifies ctx is detected and excluded from schema
• test_context_injection_callable_class: Verifies ctx is properly injected at runtime

Fixes #1974

[giulio-leone]

Closing to reduce noise — focusing on the most impactful PRs. Happy to reopen if there's interest.