fix(process_registry): poll() must not mark completion as consumed (#10156)

[someaka]

Closes #10156.

Problem

Commit f53a5a7 (#8228) added _completion_consumed tracking to suppress duplicate notify_on_complete notifications. The fix was correct for wait() (blocking call = output consumed), but was incorrectly extended to poll() (read-only query = should have no side effects).

Consequence: when the agent calls process(action='poll') on an exited background process, poll() marks the completion as consumed. The watcher thread (which checks is_completion_consumed() before injecting the notification) sees True and skips the notification. The user never sees the background process completion message.

Fix

Two changes, both in tools/process_registry.py:

1. Remove self._completion_consumed.add(session_id) from poll()
2. Fix is_completion_consumed() docstring: poll → read_log

Only wait() and read_log() should mark completion as consumed. poll() is a read-only query with no side effects.

[alt-glitch]

Duplicate of #10158 — same fix (remove _completion_consumed.add() from poll()). Previous duplicate PRs: #26616, #21903.

[someaka]

Voluntarily closing — #10158 by @de1tydev is the canonical PR with proper test coverage (30 tests). This was a duplicate.