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/ai/agent/api/impl.rs

@@ -167,13 +167,17 @@ pub async fn generate_multi_agent_output(
 
                 log::info!("[bedrock] needs_create_task={needs_create_task}");
 
-                let model_id = request
+                let mut model_id = request
                     .settings
                     .as_ref()
                     .and_then(|s| s.model_config.as_ref())
                     .map(|mc| mc.base.clone())
                     .unwrap_or_default();
 
+                if model_id.is_empty() || model_id == "auto" {
+                    model_id = "us.anthropic.claude-sonnet-4-20250514-v1:0".to_string();
+                }
+
                 log::info!("[bedrock] Model: {model_id}");
 
                 let diagnostic_logger =