fix(delegation): route async results to origin session

[dschnurbusch]

Summary

Fix async delegate_task completions in desktop/TUI so results return to the session that commissioned the subagent, even when context compression rotates the durable session id while the child is running.

• carry a live HERMES_UI_SESSION_ID / origin_ui_session_id return address with async delegation completion events
• for TUI dispatch, capture the live parent_agent.session_id instead of only the approval/session-context key
• make the desktop/TUI completion poller prefer the commissioning UI session and resolve compression continuation chains before treating an event as orphaned
• add regression coverage for stale/orphaned keys, compression continuations, finalized origin tabs, and TUI delegate dispatch capture

Why

Desktop/TUI sessions share one process-wide completion queue. If an async delegation completion only carries a durable session_key, and that key goes stale after context compression/session rotation, another session poller can consume the event and inject the delegated result into the wrong conversation.

Related PRs

I found related open PRs while checking for duplicate work: #58690, #57535, and #57586. This PR is a focused variant for the desktop/TUI path that adds a live UI return address plus compression-chain resolution rather than relying only on the durable session key.

Verification

• live smoke test after restarting the desktop app: dispatched deleg_586ecc5e; result returned to the commissioning session with DELEGATION_ROUTING_SMOKE_OK_20260706_1317
• independent review subagent passed; follow-up suggestions applied
• python -m py_compile gateway/session_context.py tools/async_delegation.py tools/delegate_tool.py tui_gateway/server.py
• python -m pytest tests/tools/test_async_delegation.py::test_delegate_task_background_routes_async_and_does_not_block tests/tools/test_async_delegation.py::test_delegate_task_background_uses_live_tui_agent_session_id tests/tools/test_async_delegation.py::test_delegate_task_background_batch_runs_as_one_unit tests/test_tui_gateway_server.py::test_notification_event_routing_by_session_key tests/test_tui_gateway_server.py::test_async_delegation_event_prefers_origin_ui_session tests/test_tui_gateway_server.py::test_notification_event_follows_compression_continuation tests/test_tui_gateway_server.py::test_finalized_origin_ui_session_falls_back_to_live_continuation tests/gateway/test_session_env.py tests/gateway/test_session_context_inheritance.py -q -o 'addopts=' → 29 passed
• python -m pytest tests/tools/test_async_delegation.py tests/gateway/test_session_env.py tests/gateway/test_session_context_inheritance.py -q -o 'addopts=' → 43 passed
• git diff --check
• GitHub CI: mergeable=MERGEABLE, mergeStateStatus=CLEAN

[teknium1]

Merged via PR #60863 — your commit was cherry-picked onto current main with your authorship preserved in git log, plus a follow-up commit adding fail-closed orphan handling (unowned delegation payloads are never adopted by a foreign session) and session-scoped delegation lifecycle (a closing session interrupts its own in-flight subagents). The origin_ui_session_id return-address design and the compression-chain resolution were exactly the right shape — thanks for the thorough cross-referencing against the sibling PRs in your body text.