feat(llm): return structured rate-limit info on 429 responses

Add RateLimitInfo interface and CompletionResult type union so callers
can distinguish 429 rate limits from other errors. Includes isRateLimited
type guard and per-request model override in CompletionRequest.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/llm/__tests__/openRouterClient.test.ts

@@ -62,11 +62,63 @@ describe('openRouterClient', () => {
     ]);
   });
 
-  it('returns null on HTTP error', async () => {
+  it('returns rateLimited result with reset timestamp on 429', async () => {
     globalThis.fetch = vi.fn().mockResolvedValue({
       ok: false,
       status: 429,
       statusText: 'Too Many Requests',
+      json: () => Promise.resolve({
+        error: {
+          code: 429,
+          message: 'Rate limit exceeded',
+          metadata: {
+            headers: {
+              'X-RateLimit-Reset': '1741305600000',
+            },
+          },
+        },
+      }),
+    });
+
+    const client = createOpenRouterClient(mockConfig);
+    const result = await client.complete({
+      system: 'sys',
+      user: 'usr',
+    });
+
+    expect(result).toEqual({
+      rateLimited: true,
+      resetAt: 1741305600000,
+    });
+  });
+
+  it('returns rateLimited result with null resetAt when 429 has no reset header', async () => {
+    globalThis.fetch = vi.fn().mockResolvedValue({
+      ok: false,
+      status: 429,
+      statusText: 'Too Many Requests',
+      json: () => Promise.resolve({
+        error: { code: 429, message: 'Rate limit exceeded' },
+      }),
+    });
+
+    const client = createOpenRouterClient(mockConfig);
+    const result = await client.complete({
+      system: 'sys',
+      user: 'usr',
+    });
+
+    expect(result).toEqual({
+      rateLimited: true,
+      resetAt: null,
+    });
+  });
+
+  it('returns null on non-429 HTTP errors', async () => {
+    globalThis.fetch = vi.fn().mockResolvedValue({
+      ok: false,
+      status: 500,
+      statusText: 'Internal Server Error',
     });
 
     const client = createOpenRouterClient(mockConfig);

server/src/llm/openRouterClient.ts

@@ -6,15 +6,27 @@ const OPENROUTER_URL = 'https://openrouter.ai/api/v1/chat/completions';
 export interface CompletionRequest extends RenderedPrompt {
   maxTokens?: number;
   temperature?: number;
+  model?: string;
+}
+
+export interface RateLimitInfo {
+  rateLimited: true;
+  resetAt: number | null;
+}
+
+export type CompletionResult = string | null | RateLimitInfo;
+
+export function isRateLimited(result: CompletionResult): result is RateLimitInfo {
+  return result != null && typeof result === 'object' && 'rateLimited' in result;
 }
 
 export interface OpenRouterClient {
-  complete(request: CompletionRequest): Promise<string | null>;
+  complete(request: CompletionRequest): Promise<CompletionResult>;
 }
 
 export function createOpenRouterClient(config: LlmConfig): OpenRouterClient {
   return {
-    async complete(request: CompletionRequest): Promise<string | null> {
+    async complete(request: CompletionRequest): Promise<CompletionResult> {
       try {
         const response = await fetch(OPENROUTER_URL, {
           method: 'POST',
@@ -24,7 +36,7 @@ export function createOpenRouterClient(config: LlmConfig): OpenRouterClient {
             'Content-Type': 'application/json',
           },
           body: JSON.stringify({
-            model: config.model,
+            model: request.model ?? config.model,
             max_tokens: request.maxTokens ?? config.maxTokens,
             temperature: request.temperature ?? config.temperature,
             messages: [
@@ -35,6 +47,16 @@ export function createOpenRouterClient(config: LlmConfig): OpenRouterClient {
         });
 
         if (!response.ok) {
+          if (response.status === 429) {
+            try {
+              const data = await response.json();
+              const resetStr = data?.error?.metadata?.headers?.['X-RateLimit-Reset'];
+              const resetAt = resetStr ? Number(resetStr) : null;
+              return { rateLimited: true, resetAt };
+            } catch {
+              return { rateLimited: true, resetAt: null };
+            }
+          }
           console.warn(`OpenRouter API error: ${response.status} ${response.statusText}`);
           return null;
         }