add constant for request cancelled and use it in case of cancelled task

Author: davemssavage

src/mcp/shared/session.py

@@ -29,6 +29,7 @@
     ServerNotification,
     ServerRequest,
     ServerResult,
+    REQUEST_CANCELLED,
 )
 
 SendRequestT = TypeVar("SendRequestT", ClientRequest, ServerRequest)
@@ -279,7 +280,7 @@ async def send_request(
                             )
 
                         raise McpError(
-                            ErrorData(code=32601, message="Request cancelled")
+                            ErrorData(code=REQUEST_CANCELLED, message="Request cancelled")
                         )
 
             except TimeoutError:

src/mcp/types.py

@@ -146,6 +146,7 @@ class JSONRPCResponse(BaseModel):
 METHOD_NOT_FOUND = -32601
 INVALID_PARAMS = -32602
 INTERNAL_ERROR = -32603
+REQUEST_CANCELLED = -32604
 
 
 class ErrorData(BaseModel):