refactor(terminal): remove check_interval parameter (#8001)

The check_interval parameter on terminal_tool sent periodic output
updates to the gateway chat, but these were display-only — the agent
couldn't see or act on them. This added schema bloat and introduced
a bug where notify_on_complete=True was silently dropped when
check_interval was also set (the not-check_interval guard skipped
fast-watcher registration, and the check_interval watcher dict
was missing the notify_on_complete key).

Removing check_interval entirely:
- Eliminates the notify_on_complete interaction bug
- Reduces tool schema size (one fewer parameter for the model)
- Simplifies the watcher registration path
- notify_on_complete (agent wake-on-completion) still works
- watch_patterns (output alerting) still works
- process(action='poll') covers manual status checking

Closes #7947 (root cause eliminated rather than patched).

tests/tools/test_code_execution.py

@@ -380,7 +380,7 @@ class TestStubSchemaDrift(unittest.TestCase):
     # Parameters that are internal (injected by the handler, not user-facing)
     _INTERNAL_PARAMS = {"task_id", "user_task"}
     # Parameters intentionally blocked in the sandbox
-    _BLOCKED_TERMINAL_PARAMS = {"background", "check_interval", "pty", "notify_on_complete"}
+    _BLOCKED_TERMINAL_PARAMS = {"background", "pty", "notify_on_complete"}
 
     def test_stubs_cover_all_schema_params(self):
         """Every user-facing parameter in the real schema must appear in the