Two client-side halves of the #55578 session split:
1. Submit with a null activeSessionId but a SELECTED stored session now
resumes that stored session instead of falling straight through to
createBackendSessionForSend - which silently forked the user's
conversation into a brand-new session that then got orphan-reaped.
New-chat drafts (no stored selection) still create sessions as before.
2. prompt.submit recovery now also fires on gateway request timeouts,
not only 'session not found'. A starved backend loop (the async-
delegation poller spin) rejects the submit with 'request timed out'
even though the stored session is fine; previously that surfaced an
error, left the binding cleared, and set up the split on the next
send.
Fail-then-pass: 2 new tests fail with production code reverted.
8e734810df