fix: add explicit type annotation for call_tool decorator

Replace `Callable[..., Awaitable[...]]` with a concrete `CallToolHandler`
type alias that specifies the exact expected function signature
`(str, dict[str, Any]) -> Awaitable[...]`.

This enables static type checkers (mypy/pyright) to validate decorated
functions without requiring `# type: ignore` comments, making the
`py.typed` marker actually useful.

Fixes #1822

Github-Issue: #1822

Author: Jay Hemnani

src/mcp/server/lowlevel/server.py

@@ -105,6 +105,14 @@ async def main():
 UnstructuredContent: TypeAlias = Iterable[types.ContentBlock]
 CombinationContent: TypeAlias = tuple[UnstructuredContent, StructuredContent]
 
+# type alias for call_tool handler function signature
+CallToolHandler: TypeAlias = Callable[
+    [str, dict[str, Any]],
+    Awaitable[
+        UnstructuredContent | StructuredContent | CombinationContent | types.CallToolResult | types.CreateTaskResult
+    ],
+]
+
 # This will be properly typed in each Server instance's context
 request_ctx: contextvars.ContextVar[RequestContext[ServerSession, Any, Any]] = contextvars.ContextVar("request_ctx")
 
@@ -497,18 +505,7 @@ def call_tool(self, *, validate_input: bool = True):
         If outputSchema is defined, validates structuredContent or errors if missing.
         """
 
-        def decorator(
-            func: Callable[
-                ...,
-                Awaitable[
-                    UnstructuredContent
-                    | StructuredContent
-                    | CombinationContent
-                    | types.CallToolResult
-                    | types.CreateTaskResult
-                ],
-            ],
-        ):
+        def decorator(func: CallToolHandler) -> CallToolHandler:
             logger.debug("Registering handler for CallToolRequest")
 
             async def handler(req: types.CallToolRequest):