fix(watchdog): persist UI-saved Check Interval across restarts (#10601)

[localai-bot]

Description

Fixes #10601.

The watchdog Check Interval configured via the Settings UI (POST /api/settings) reverted to 500ms on every container restart, even though it was correctly written to runtime_settings.json. The other watchdog settings (idle/busy enable + timeouts) persisted fine.

Root cause

NewApplicationConfig baseline-defaulted WatchDogInterval to 500ms, whereas the idle/busy timeouts default to 0. The startup loader loadRuntimeSettingsFromFile applies a persisted value only when the field is still at its zero default - the heuristic it uses to mean "this wasn't set by an env var" (so env/CLI keeps precedence over the file). Because the interval was always 500ms at that point, the guard options.WatchDogInterval == 0 never held, the persisted value was never read back, and the interval reset to the default on every boot.

The idle/busy timeouts work precisely because they are not baseline-defaulted (they stay 0 until env/file sets them), so their guard fires.

Fix

• Drop the non-zero baseline default in NewApplicationConfig so the interval behaves like its sibling timeouts (0 = unset, file value loads).
• Supply the effective 500ms default at the watchdog layer instead: WithWatchdogInterval now ignores a non-positive value, preserving DefaultWatchDogOptions' 500ms and ensuring a 0 interval can never turn the watchdog loop into a busy spin.
• Mirror the interval in the live config-file watcher alongside the idle/busy timeouts (parity for direct file edits).
• Report the real 500ms default (not the stale "2s") from ToRuntimeSettings.

Env/CLI precedence is preserved: an explicitly set interval still wins over the persisted file value, consistent with the idle/busy timeouts.

Tests

Added two specs in core/application/runtime_settings_branding_test.go (constructed via NewApplicationConfig so they observe the real boot-time default):

• a UI-saved watchdog_interval is loaded on the next startup (red before the fix - reverted to 500ms);
• an explicit env/CLI interval is not overridden by the persisted file value.

core/config, pkg/model, core/application, and core/http/endpoints/localai suites pass; golangci-lint --new-from-merge-base=master is clean.

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