fix: handle CancelledError in MCP tool calls to prevent process crash

MCP SDK's anyio cancel scopes can leak CancelledError on timeout or
failure paths. Since CancelledError is a BaseException (not Exception),
it escapes both MCPToolWrapper.execute() and ToolRegistry.execute(),
crashing the agent loop.

Now catches CancelledError and returns a graceful error to the LLM,
while still re-raising genuine task cancellations from /stop.
Also catches general Exception for other MCP failures (connection
drops, invalid responses, etc.).

Related: #1055

nanobot/agent/tools/mcp.py

@@ -34,7 +34,7 @@ class MCPToolWrapper(Tool):
     def parameters(self) -> dict[str, Any]:
         return self._parameters
 
-    async def execute(self, **kwargs: Any) -> str:
+async def execute(self, **kwargs: Any) -> str:
         from mcp import types
         try:
             result = await asyncio.wait_for(
@@ -44,13 +44,24 @@ class MCPToolWrapper(Tool):
         except asyncio.TimeoutError:
             logger.warning("MCP tool '{}' timed out after {}s", self._name, self._tool_timeout)
             return f"(MCP tool call timed out after {self._tool_timeout}s)"
+        except asyncio.CancelledError:
+            # MCP SDK's anyio cancel scopes can leak CancelledError on timeout/failure.
+            # Re-raise only if our task was externally cancelled (e.g. /stop).
+            task = asyncio.current_task()
+            if task is not None and task.cancelling() > 0:
+                raise
+            logger.warning("MCP tool '{}' was cancelled by server/SDK", self._name)
+            return f"(MCP tool call was cancelled)"
+        except Exception as exc:
+            logger.warning("MCP tool '{}' failed: {}: {}", self._name, type(exc).__name__, exc)
+            return f"(MCP tool call failed: {type(exc).__name__})"
         parts = []
         for block in result.content:
             if isinstance(block, types.TextContent):
                 parts.append(block.text)
             else:
                 parts.append(str(block))
-        return "\n".join(parts) or "(no output)"
+        return "\n".join(parts) or "(no output)
 
 
 async def connect_mcp_servers(