fix(model): track intentional stops, stop misreading clean shutdowns as crashes

[richiejp]

Description

Track when we intentionally kill a backend and use that to decide if its death is unexpected.
Otherwise we get error messages because the backend process returned a non-zero status we didn't
expect and there are no standards for status codes so we'd have to ensure every backend handles signals
in the same way and explicitly uses the exit codes we expect.

Notes for Reviewers

Two separate issues made graceful backend shutdown look ungraceful in the
logs, even though the processes were being terminated correctly
(go-processmanager defaults to process-group SIGTERM + 15s grace + SIGKILL):

1. "failed to read PID" — startProcess registers a per-process graceful-
   termination handler that calls Stop(), but StopAllGRPC (registered
   earlier, via app.Shutdown) already stopped and released store-tracked
   backends first. The second Stop() then failed reading the removed
   pidfile. Guard the handler with IsAlive() so it skips already-stopped
   processes; it still covers backends StopAllGRPC doesn't track (worker-
   supervised ones).

2. "Backend process exited unexpectedly" exitCode=-1 — the exit watcher
   treated only exit codes 0/143 as clean. But a child killed by our own
   SIGTERM/SIGKILL is reported by Go as exitCode -1 (signal termination),
   not the shell's 128+signal convention, so every intentional stop logged
   a false crash warning. The exit code can't distinguish an intended stop
   from a signal-induced crash.

Track intent directly instead: a stoppingProcs sync.Map (keyed by the
*process.Process pointer) is marked wherever LocalAI calls Stop() on
purpose, and the exit watcher uses it to pick the log level — Info
"stopped" when intentional, Warn "exited unexpectedly" otherwise (still
catching real crashes). The raw exit code is reported as a field but no
longer interpreted.

Assisted-by: Claude:claude-opus-4-8 [Claude Code]
Signed-off-by: Richard Palethorpe io@richiejp.com

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