Closes #3255

The @mcp.tool, @mcp.prompt, and @mcp.resource decorator overloads on FastMCP and LocalProvider still declared FunctionTool, FunctionPrompt, and Resource | ResourceTemplate as their return types — left over from when decorator_mode="object" was the default. Since v3 defaults to "function" mode (decorators return the original function unchanged), type checkers saw these as non-callable objects and flagged any direct call to the decorated function.

The standalone decorators (from fastmcp.tools import tool, etc.) already used a TypeVar F pattern to preserve the original function type. This applies the same pattern to the method decorators:

mcp = FastMCP("demo")

@mcp.tool
def add(a: int, b: int) -> int:
    return a + b

# Type checkers now correctly see `add` as (int, int) -> int
add(a=1, b=2)  # no more type error