fix(router): align fallback semantics and oauth provider behavior

src/gateway/handlers/handlers.test.ts

@@ -1497,7 +1497,7 @@ describe('config handlers', () => {
       },
       models: {
         default: { provider: 'anthropic' as const, model: 'claude-3-haiku', api_key: 'sk-secret-key' },
-        fallback_chain: ['anthropic'],
+        fallback_chain: [],
       },
       backends: { claude_code: { enabled: false }, opencode: { enabled: false }, native: { enabled: true } },
       hooks: { confirm: ['shell.exec'], log: [], silent: [] },
@@ -1868,7 +1868,7 @@ describe('redactConfig – comprehensive credential redaction', () => {
         },
         complex: { provider: 'anthropic' as const, model: 'claude-opus', auth_token: 'at-complex' },
         local: { provider: 'ollama' as const, model: 'llama3' },
-        fallback_chain: ['anthropic'],
+        fallback_chain: [],
         local_providers: {
           ollama: { provider: 'ollama' as const, model: 'llama3', api_key: 'lp-key', auth_token: 'lp-token',
             fallback: { provider: 'llamacpp' as const, model: 'llama', api_key: 'lp-fb-key' },
@@ -2012,7 +2012,7 @@ describe('redactConfig – comprehensive credential redaction', () => {
     // models
     expect(getPath(result, 'models', 'default', 'provider')).toBe('anthropic');
     expect(getPath(result, 'models', 'default', 'model')).toBe('claude');
-    expect(getPath(result, 'models', 'fallback_chain')).toEqual(['anthropic']);
+    expect(getPath(result, 'models', 'fallback_chain')).toEqual([]);
     // web_search
     expect(getPath(result, 'web_search', 'provider')).toBe('brave');
     expect(getPath(result, 'web_search', 'max_results')).toBe(5);