fix(gateway): keep launchd KeepAlive unconditional for macOS resilience

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Problem

On macOS, when the gateway exits cleanly (e.g. during hermes gateway stop or a controlled restart), the current KeepAlive policy {SuccessfulExit: false} does NOT restart it — launchd treats exit code 0 as "intentional" and leaves the service dead.

Change

Replace:

<key>KeepAlive</key>
<dict>
    <key>SuccessfulExit</key>
    <false/>
</dict>

With:

<key>KeepAlive</key>
<true/>

This ensures launchd always restarts the gateway after any exit, matching the expectation that the service runs continuously.

Verification

Tested on macOS 26.5: after applying this change, hermes gateway stop properly unloads via bootout (which removes the service definition), while a crash or SIGTERM without bootout correctly respawns the process.

[alt-glitch]

Duplicate of #14001 and #9689 (both open) — same fix: unconditional KeepAlive in launchd plist. Also previously submitted as #29111 (closed). Root issue: #9659.

Consider closing in favor of whichever of #14001 or #9689 is rebased first.

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🔔 Bumping — this PR is still relevant and ready for review.

Recent context:

• Just opened fix(gateway): use via_service for macOS restart to avoid KeepAlive race #37063 (via_service for macOS restart) which complements this PR
• Just opened fix(gateway): drop _HERMES_GATEWAY blanket guard for legit internal restarts #37057 (drop _HERMES_GATEWAY guard) which unblocks the restart path
• Rebased to current origin/main today, no conflicts

Behavior after all three merge:

• launchd KeepAlive unconditional (this PR) → respawn on any exit
• via_service=True on macOS → clean exit, launchd restarts natively, no detached watcher race
• _HERMES_GATEWAY guard dropped → manual restart from WeChat/Telegram works

Happy to address review feedback or split into smaller PRs if preferred.