Adversarial review of the previous commit found a leak it introduced.
`attach_terminal()` subscribes and only then are the response headers written.
A client that dropped in that instant makes `end_headers()` raise
BrokenPipeError/ConnectionResetError, and because the `try/finally` started
*after* those writes, the exception escaped `_handle_terminal_output` with the
queue still in `_subscribers`. That pins `unwatched_since` at None — only
`unsubscribe()` on an empty subscriber list resets it — so the terminal became
permanently unreapable: the exact fd/thread leak (#4633) this reaper exists to
prevent, and strictly worse than the TOCTOU the previous commit fixed, since
that at least still allowed cleanup.
The try now starts immediately after the attach, so every exit path
unsubscribes. Regression test drives a handler whose `end_headers()` raises and
asserts the subscriber list is empty, `unwatched_since` is restored, and the
terminal is reapable again once the grace elapses. It fails against the
previous ordering.
ai-ag2026
a31cdb7765