fix(gateway): use unconditional KeepAlive in launchd plist so gateway restarts after clean --replace exit (#37388)

[Tranquil-Flow]

What does this PR do?

Changes the launchd plist from KeepAlive.SuccessfulExit=false to unconditional <key>KeepAlive</key><true/> so launchd restarts the gateway on ALL exit codes — including clean exit (0) from --replace. Previously, a clean --replace exit left the gateway down indefinitely.

Also updates the stale _graceful_restart_via_sigusr1 docstring that documented the old SuccessfulExit=false behavior.

Related Issue

Fixes #37388

Type of Change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• tests/hermes_cli/test_gateway_service.py — 130 passed (6 pre-existing systemd env failures unrelated)
• Fail-without-fix verified by reviewer
• Reviewer improvement pass applied — updated docstring, expanded test assertions

Root Cause

generate_launchd_plist() used KeepAlive.SuccessfulExit=false. When --replace triggered a clean exit (code 0), launchd did NOT restart — gateway stayed down.

Fix

Change to unconditional <key>KeepAlive</key><true/> — launchd always restarts regardless of exit code, matching systemd Restart=always.

Checklist

• My code follows the code style of this project
• I have added tests that prove my fix is effective
• All new and existing tests pass locally
• No new dependencies added
• No AI attribution in commits

[teknium1]

Fixed via PR #39000 (#39000), merged to main. You submitted the first fix for this issue and identified the correct approach — credited in the merged PR. The salvaged implementation (#38796) was selected for its test + comment polish, but your diagnosis was spot on. Thanks!