fix(gateway): reap orphaned gateways before spawning restart (#77276)

[RelaxJonh]

Problem

When the desktop app restarts, the old serve process exits but its gateway child gets reparented to launchd (PPID=1) and keeps its platform connection alive. The new serve spawns a fresh gateway, resulting in two live gateways racing the same connection — messages split across parallel session trees (#77276).

Root cause

_reap_unsupervised_gateway_orphans() was implemented for the CLI restart path (stop_profile_gateway() → #75936) but the dashboard's _spawn_gateway_restart() path was not covered.

Fix

Call _reap_unsupervised_gateway_orphans() at the top of _spawn_gateway_restart() before spawning the new hermes gateway restart child. The reap is wrapped in a try/except so a failure never blocks the restart.

Testing

• New unit tests verify the reap is called before spawn
• Existing test_gateway.py orphan tests continue to pass
• Lint clean

Fixes #77276

[teknium1]

Merged via PR #83542 — your commit(s) were cherry-picked onto current main with your authorship preserved in git log (rebase merge). Thank you for the contribution!

This follow-up PR completed the EMFILE hardening cluster after #83406: restart-path gateway orphan reap, Desktop-managed gateway termination on serve shutdown, SSH-spawn ulimit raise, and the dashboard iterdir→scandir fd-leak fixes. Everything was live-tested end-to-end on a real serve backend before merge, including a hostile unreadable-profile-dir fixture that surfaced (and fixed) a pre-existing /api/profiles 500 along the way.