fix: suppress duplicate completion notifications when agent already consumed output via wait/poll/log (#8228)

When the agent calls process(action='wait') or process(action='poll')
and gets the exited status, the completion_queue notification is
redundant — the agent already has the output from the tool return.
Previously, the drain loops in CLI and gateway would still inject
the [SYSTEM: Background process completed] message, causing the
agent to receive the same information twice.

Fix: track session IDs in _completion_consumed set when wait/poll/log
returns an exited process. Drain loops in cli.py and gateway watcher
skip completion events for consumed sessions. Watch pattern events
are never suppressed (they have independent semantics).

Adds 4 tests covering wait/poll/log marking and running-process
negative case.

gateway/run.py

@@ -7009,7 +7009,9 @@ class GatewayRunner:
 
             if session.exited:
                 # --- Agent-triggered completion: inject synthetic message ---
-                if agent_notify:
+                # Skip if the agent already consumed the result via wait/poll/log
+                from tools.process_registry import process_registry as _pr_check
+                if agent_notify and not _pr_check.is_completion_consumed(session_id):
                     from tools.ansi_strip import strip_ansi
                     _out = strip_ansi(session.output_buffer[-2000:]) if session.output_buffer else ""
                     synth_text = (