* refactor(cookbook): move _diagnose_serve_output to module level in cookbook_helpers
Extracts the nested _diagnose_serve_output function from inside
setup_cookbook_routes() and moves it to module level in cookbook_helpers.py,
alongside the other helper functions it logically belongs with.
No behaviour change — the function is now importable directly for testing
and by other callers without going through the route factory closure.
* fix(cookbook): surface backend diagnosis when serve fails in background
The background poll (_pollBackgroundStatus) already received `diagnosis`
and `cmd` from /api/cookbook/tasks/status but discarded both. When a serve
job died while the Cookbook modal was closed, reopening it showed only a
red error badge with no context.
- Persist live.diagnosis into task._backendDiagnosis in localStorage so it
survives modal close/reopen and page refresh
- Persist live.cmd into task.payload._cmd for agent-spawned tasks so the
crash report includes the actual command
- After _renderRunningTab(), walk rendered cards and call _showDiagnosis()
for any that have a stored _backendDiagnosis but no panel yet
- In _renderTaskCard(), use _backendDiagnosis as a fallback when the
client-side _terminalServeDiagnosis() finds nothing
* test(cookbook): add coverage for _diagnose_serve_output error patterns
10 tests verifying the 16 serve-failure patterns:
- CUDA OOM, port-in-use, vLLM missing, gated model
- Traceback fallback fires without startup success marker
- Traceback suppressed when server actually started
- Clean/empty output returns None
- trust-remote-code and no-GGUF patterns
