Fix stale thinking indicators for tagged admin agents

[pkuntong]

Explanation of Change

When an admins-room request tags a specific agent, the backend can still return stale “thinking” labels for other agent participants. The UI now latches the account IDs mentioned on the current user request for that processing cycle and filters stale labels to the tagged agents. Untagged admins requests continue to show all active agents, and regular Concierge chats are unchanged.

This also adds a selector that finds the latest user comment by actor and unit coverage for tagged and untagged admins requests.

Fixed Issues

$ #91700
PROPOSAL: #91700 (comment)

Tests

1. Added unit coverage for tagged admins requests with a stale label for another agent.
2. Added unit coverage confirming untagged admins requests show Concierge plus every active agent.
3. npx --yes oxfmt@0.55.0 --check src/pages/inbox/AgentZeroStatusContext.tsx src/selectors/ReportAction.ts tests/unit/AgentZeroStatusContextTest.ts — passed.
4. git diff --check — passed.
5. npm run lint-changed — blocked because dependencies are not installed locally (@eslint/eslintrc missing).

Offline tests

No network or offline behavior changed.

QA Steps

1. In an admins room with multiple agents, tag one agent and send a request.
2. Verify only the tagged agent shows a thinking indicator while the response is processing.
3. Send an untagged admins request and verify all active agents plus Concierge can show thinking indicators.
4. Verify a regular Concierge chat is unchanged.