Fix accidental line corruption in split_message() where 'break' was
merged with unrelated code during manual editing.
The actual fix: build_assistant_message() now returns content or ""
instead of content (which could be None), preventing providers like
MiMo V2 Omni from rejecting tool-call messages with missing text field.
Fixes#2519
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
Alfredo ArenasandXubin Ren
