fix(backends): validate backends.default and enabled backend set

src/daemon/index.test.ts

@@ -1,7 +1,7 @@
 import { describe, expect, it } from 'vitest';
 
 import { configSchema } from '../config/schema.js';
-import { createConfiguredExternalBackend, createConfiguredExternalBackends } from './index.js';
+import { createConfiguredExternalBackend, createConfiguredExternalBackends, validateBackendConfig } from './index.js';
 
 describe('createConfiguredExternalBackend', () => {
   const base = configSchema.parse({
@@ -69,3 +69,38 @@ describe('createConfiguredExternalBackend', () => {
     expect(configured.backends.gemini?.name).toBe('gemini');
   });
 });
+
+describe('validateBackendConfig', () => {
+  const base = configSchema.parse({
+    telegram: { bot_token: 'test', allowed_chat_ids: [1] },
+    models: { default: { provider: 'anthropic', model: 'claude-3' } },
+  });
+
+  it('throws when no backend is enabled', () => {
+    const cfg = {
+      ...base,
+      backends: {
+        ...base.backends,
+        native: { enabled: false },
+        codex: { ...base.backends.codex, enabled: false },
+        claude_code: { ...base.backends.claude_code, enabled: false },
+        opencode: { ...base.backends.opencode, enabled: false },
+        gemini: { ...base.backends.gemini, enabled: false },
+      },
+    };
+    expect(() => validateBackendConfig(cfg)).toThrow('No backend enabled');
+  });
+
+  it('throws when backends.default points to a disabled backend', () => {
+    const cfg = {
+      ...base,
+      backends: {
+        ...base.backends,
+        default: 'gemini' as const,
+        codex: { ...base.backends.codex, enabled: true },
+        gemini: { ...base.backends.gemini, enabled: false },
+      },
+    };
+    expect(() => validateBackendConfig(cfg)).toThrow('backends.default=gemini is not enabled');
+  });
+});