fix(delegation): route async delegate_task results back to originating session

[asimons81]

Summary

Fixes #58684.

Background delegate_task results are delivered to whichever session happens to be active when the subagent finishes, not the session that dispatched it — causing cross-contamination between unrelated sessions.

Root cause: The completion event already carries the dispatching session's session_key (captured at delegate_tool.py:2798), but the delivery router ignored it. drain_notifications() consumed all events from the shared queue without filtering by session, and the CLI/TUI post-turn drains had no session awareness.

Changes

• tools/process_registry.py: drain_notifications() now accepts an optional session_key parameter. When provided, async-delegation events whose session_key doesn't match are re-queued instead of consumed.
• cli.py: The process_loop drain passes the active session key to drain_notifications()
• tui_gateway/server.py: The post-turn drain passes the TUI session's session_key
• gateway/run.py: _build_process_event_source logs a warning when routing metadata is unresolvable (previously a silent drop)
• Tests: two new regression tests covering session-filtered and unfiltered drain behavior

Behavior

Scenario	Before	After
Dispatch from session A, result arrives while session A is active	✅ Works	✅ Unchanged
Dispatch from session A, switch to session B before result arrives	❌ Result goes to session B	✅ Result waits for session A's drain
No session_key (CLI default)	✅ All events consumed	✅ Unchanged (backward compatible)
Gateway event with unresolvable session key	🤫 Silently dropped	⚠️ Warning logged

Testing

• test_drain_notifications_filters_async_delegation_by_session_key (new) — verifies events are correctly filtered and re-queued per session
• test_drain_notifications_no_filter_passes_all_async_delegation (new) — backward compatibility: no filter = all events consumed
• All 5 drain_notifications tests pass
• 19/20 async_delegation tests pass (1 pre-existing unrelated failure in test_crashed_runner_produces_error_completion)
• 35/35 notify_on_complete tests pass

[teknium1]

Merged via PR #60869 — your commit was cherry-picked onto current main with your authorship preserved in git log, plus a follow-up commit extending your session_key filter with a positive-proof ownership callback (compression-chain aware, fail-closed) for the TUI drain path. Your two-session regression tests transferred cleanly and pinned the behavior. Thanks — the drain hole was the piece none of the poller-side fixes touched.