pty_ws already fell back to the per-channel active-session file when a
/chat WS connects with no ?resume= param, replaying the whole session
into the PTY, but the frontend only pinned xterm's viewport to the
bottom when resumeParam came from the URL (#59591). The implicit path
had no way to learn a replay was happening, so the viewport stayed at
the top of the scrollback.
pty_ws now sends a one-off JSON control frame naming the session id it
resolved from the active-session file, before any PTY bytes; PTY
output itself always arrives as binary frames, so this is unambiguous
on the wire. ChatPage tracks an `effectiveResume` value seeded from
resumeParam and updated when this control frame arrives, and the
existing follow-scroll/sanitizer/hydration logic keys off it instead
of the URL param alone.
Fixes#93518.
chelsealong
9c013eaaf8