fix(tui): route completion notifications to owner live session

[LeonSGP43]

Summary

• route TUI background-process completion notifications to the live session that owns the process instead of whichever per-session poller drained the global queue first
• factor the poller delivery path so both the steady-state loop and shutdown drain use the same owner-aware routing logic
• add a regression test covering two concurrent live TUI sessions where session B's poller drains session A's completion event

Closes #35652.

Why

_notification_poller_loop() already documents that completion_queue is global, but it always emitted the notification and synthetic turn back to the current poller sid. In a multi-live-session TUI, that let session B render and ingest session A's [IMPORTANT: Background process ... completed] message.

This change keeps the existing queue model but resolves the event back to the owning live session via the tracked process session_key, then injects the notification into that owner session.

Scope

• In scope: owner-scoped routing for TUI completion/watch notifications once an event has been dequeued
• Out of scope: the older core-side spawn/session-id capture family covered by PR fix(tools): env-immune spawn-owner session id so background-task wakeups cannot be misrouted across concurrent sessions #28560; this change only fixes the TUI multi-live-session drain race in tui_gateway/server.py

Validation

• python -m pytest tests/test_tui_gateway_server.py -k 'notification_poller' -o 'addopts=' -q
• python -m ruff check tui_gateway/server.py tests/test_tui_gateway_server.py
• git diff --check

[teknium1]

Closing — superseded. Main has since gained _notification_event_belongs_elsewhere (per-session ownership filtering on the shared completion queue), and PR #60863 completed the design with an origin-session return address stamped at dispatch, compression-chain-aware ownership, and fail-closed handling for unowned payloads. Your owner-directed routing idea was the right instinct — the landed design achieves the same guarantee through positive-proof ownership at the consumer. Thanks!