test: harden timing-fragile test and add cross-tool ContextVar isolation test

Replace asyncio.sleep(0.05) with an asyncio.Event + patched Lock.acquire
to guarantee the waiting task has reached the lock before asserting.  Add
a test confirming LongTaskTool and CompleteGoalTool ContextVars are
isolated, and document the design intent in _GoalToolsMixin.

nanobot/agent/tools/long_task.py

@@ -46,6 +46,9 @@ class _GoalToolsMixin(ContextAware):
     def __init__(self, sessions: SessionManager, bus: Any | None = None) -> None:
         self._sessions = sessions
         self._bus = bus
+        # Each subclass gets its own ContextVar so concurrent tasks across
+        # different tool types (LongTaskTool vs CompleteGoalTool) do not
+        # interfere with each other.
         self._request_ctx: ContextVar[RequestContext | None] = ContextVar(
             f"{self.__class__.__name__}_request_ctx",
             default=None,