feat(providers): make provider request timeout configurable (default 30s) (#1400)

The provider verify call and the provider models-list call used hardcoded
timeouts (30s and 15s) that were too low for slow models. Introduce a
tenant-scoped, UI-editable setting `providers.request_timeout_sec`
(default 30) stored in system_configs, consumed by both handlers, and
overlaid from config.json like tts.timeout_ms. Adds a field to the System
Settings modal (all 4 locales) so operators can raise it without a
redeploy.

Co-authored-by: Bruno Clermont <bruno.clermont@gmail.com>

cmd/gateway_system_config_sync.go

@@ -127,6 +127,9 @@ func seedConfigForContext(ctx context.Context, sc store.SystemConfigStore, cfg *
 	setInt("tools.browser.max_pages", cfg.Tools.Browser.MaxPages)
 	set("tools.browser.cookie_sync_enabled", fmt.Sprintf("%t", cfg.Tools.Browser.CookieSyncEnabled))
 
+	// Providers
+	setInt("providers.request_timeout_sec", cfg.Providers.RequestTimeoutSec)
+
 	// TTS
 	set("tts.provider", cfg.Tts.Provider)
 	set("tts.auto", cfg.Tts.Auto)