fix(process_registry): keep poll() read-only — do not mark completions consumed

[someaka]

Summary

Keeps process_registry.poll() read-only for exited background processes.

poll() should report status, not mark a notify_on_complete result as consumed. Otherwise a normal status check can race with the gateway watcher and permanently suppress the synthetic completion event before it is delivered back to the chat.

Root cause

PR #8228 (f53a5a7f) added _completion_consumed tracking to avoid duplicate completion notifications. That behavior is correct for explicit consumption paths like wait() and read_log(), but not for poll():

• wait() = blocking consume ("give me the result, I'm done")
• read_log() = explicit consumption ("I read the output, I'm done")
• poll() = read-only status query ("what's the current state?")

Treating poll() as consumption can cause a race:

1. Agent starts a background process with notify_on_complete=true
2. Gateway creates a watcher asyncio task (polls every 5s)
3. Process exits
4. Agent calls process(action='poll') on the next turn
5. poll() marks _completion_consumed.add(session_id) (the bug)
6. Watcher wakes up → checks is_completion_consumed() → returns True → skips notification → breaks (never retries)

Changes

• tools/process_registry.py: removed self._completion_consumed.add(session_id) from poll()
• tests/tools/test_process_registry.py: added test_poll_does_not_mark_completion_consumed regression guard

Test plan

python -m pytest tests/tools/test_process_registry.py -x -q
65 passed in 3.86s

Closes #10156

[alt-glitch]

Duplicate of #10158 (same fix for #10156). Previous duplicates by same author: #26659 (closed), also #21903 (closed).

[someaka]

Closing — duplicate of #10158 per @alt-glitch.