Avoid blocking TUI on agent metadata hydration

[etraut-openai]

Why

Fixes #16688.

The TUI currently hydrates collab receiver metadata by awaiting thread/read before each active-thread notification is rendered. During large subagent fan-outs, the embedded app-server can be busy starting agents and processing spawn work, so those synchronous metadata reads queue behind the fan-out and block the TUI event loop. That makes the UI appear frozen even though the underlying agent work can continue.

What Changed

• Replaced eager thread/read metadata hydration on the active notification path with local receiver-thread caching.
• Kept ThreadStarted and picker refreshes as the places that fill in agent nickname/role metadata when it is available.
• Skipped caching receiver threads that are explicitly reported as NotFound, avoiding live-looking ghost entries for failed stale-agent calls.
• Added TUI tests covering both local receiver caching and NotFound suppression.

Verification

• cargo test -p codex-tui collab_receiver_notification
• just fix -p codex-tui

I also ran the full cargo test -p codex-tui; the new test passed, but the full process later aborted with an unrelated stack overflow in tests::fork_last_filters_latest_session_by_cwd_unless_show_all.