Fix Bedrock not being used: enable by default, remove model_id gating, set fallback default model

- bedrock_enabled default: false -> true (was never routing to Bedrock)
- Remove is_bedrock_model check from bedrock_config_if_applicable (model_id
  could be 'auto' from server-populated prefs, causing Bedrock to be skipped)
- Default to claude-sonnet-4 when model_id is empty or 'auto'
- fallback_to_warp default: true -> false (no server exists)
- Smoke test: exit immediately on no-text with diagnostic info instead of
  polling forever

app/src/settings/ai.rs

@@ -1071,7 +1071,7 @@ define_settings_group!(AISettings, settings: [
     // Whether direct Bedrock integration is enabled (client calls Bedrock API directly).
     bedrock_enabled: BedrockEnabled {
         type: bool,
-        default: false,
+        default: true,
         supported_platforms: SupportedPlatforms::DESKTOP,
         sync_to_cloud: SyncToCloud::Globally(RespectUserSyncSetting::Yes),
         private: false,
@@ -1113,7 +1113,7 @@ define_settings_group!(AISettings, settings: [
     // Whether to fall back to routing through the Warp server when Bedrock credentials fail.
     bedrock_fallback_to_warp: BedrockFallbackToWarp {
         type: bool,
-        default: true,
+        default: false,
         supported_platforms: SupportedPlatforms::DESKTOP,
         sync_to_cloud: SyncToCloud::Globally(RespectUserSyncSetting::Yes),
         private: false,