fix(update): don't report success when the Desktop rebuild failed (supersedes #88359, #87984)

[OutThisLife]

What does this PR do?

Supersedes #88359 and #87984.

hermes update treated a failed Desktop pack as non-fatal: it printed an early warning, then still ended with ✓ Update complete!. Python moved on; Hermes.exe stayed on the previous build. That is the summary half of #88251 (the get-windows binding recovery is #88233).

_rebuild_desktop_after_update now returns False only when a rebuild was attempted and failed. Both the git and ZIP paths then print ⚠ Update partially complete instead of the success banner, and gateway mode writes 1 to .update_exit_code so /update watchers do not claim success.

The success-path output is unchanged when the Desktop rebuild succeeds or is not needed.

Credit: @JoaoMarcos44 (exit status / withhold the banner) and @liuhao1024 (return-value contract and tests).

Related Issue

Related to #88251. Supersedes #88359, #87984.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✅ Tests (adding or improving test coverage)

Changes Made

• hermes_cli/update_cmd.py: _rebuild_desktop_after_update() returns bool; shared _print_update_summary() and _write_gateway_update_exit_code().
• Git, ZIP, and ZIP-fallback paths withhold ✓ Update complete! when the rebuild failed.
• Gateway mode writes .update_exit_code=1 in that case.
• tests/hermes_cli/test_update_desktop_stale_warning.py: rebuild contract, banner, combined node+desktop failure, exit-code file.

How to Test

1. scripts/run_tests.sh tests/hermes_cli/test_update_desktop_stale_warning.py tests/hermes_cli/test_cmd_update.py tests/hermes_cli/test_update_version_report.py -q — 8 new tests plus existing update coverage.
2. Force a desktop pack failure and run hermes update — the run should end with ⚠ Update partially complete and a retry hint, not ✓ Update complete!.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS 15

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

[OutThisLife]

Kicking CI: the ci.yml run died before scheduling any jobs (workflow-file-issue, 0 jobs, cannot retry). Reopening to retrigger.