feat: AI 调用稳定性增强（可重试错误分类 + 指数退避 + 修复链路错误透传）

- providers/base.ts 引入 ApiRequestError 与 isRetryableError，按 429/5xx/超时/网络错误/401 分类
- chatWithRetry 升级为最多 3 次指数退避（1s/2s/4s），非重试错误立即上抛；新增 setRetryBaseDelayForTest 钩子
- aiFixEngine/customFixEngine 不再吞错，修复失败原因透传为 ai-error: <原因>
- 扩展 ai-empty-response/customFixEngine 测试覆盖重试与分类

src/ai/engine.ts

@@ -1,5 +1,5 @@
 import * as vscode from 'vscode';
-import { EmptyContentError } from './providers/base';
+import { isRetryableError } from './providers/base';
 import type { AIProvider, ChatOptions } from './providers/base';
 import { createProvider } from './factory';
 import { getAIProvider, getAIModel, getAIBaseUrl, getAITemperature, getAITimeout, getAIMaxTokens, getAIOutputLanguage, getApiKey } from '../config';
@@ -83,20 +83,36 @@ export function parseJsonResponse(raw: string): object {
   }
 }
 
+const RETRY_MAX_ATTEMPTS = 3;
+const RETRY_BASE_DELAY_MS = 1000;
+
+let retryBaseDelayMs = RETRY_BASE_DELAY_MS;
+
+export function setRetryBaseDelayForTest(ms: number): void {
+  retryBaseDelayMs = ms;
+}
+
+function sleep(ms: number): Promise<void> {
+  return new Promise(resolve => setTimeout(resolve, ms));
+}
+
 export async function chatWithRetry(
   provider: AIProvider,
   systemPrompt: string,
   userPrompt: string,
   options: ChatOptions
 ): Promise<string> {
-  try {
-    return await provider.chat(systemPrompt, userPrompt, options);
-  } catch (err) {
-    if (err instanceof EmptyContentError) {
-      return provider.chat(systemPrompt, userPrompt, options);
+  for (let attempt = 0; attempt <= RETRY_MAX_ATTEMPTS; attempt++) {
+    try {
+      return await provider.chat(systemPrompt, userPrompt, options);
+    } catch (err) {
+      if (attempt === RETRY_MAX_ATTEMPTS || !isRetryableError(err)) {
+        throw err;
+      }
+      await sleep(retryBaseDelayMs * 2 ** attempt);
     }
-    throw err;
   }
+  throw new Error('unreachable');
 }
 
 function buildCustomRuleSystemPrompt(): string {