fix: inline test-api.html as TypeScript string export for Vercel

Bun import of .html files returns HTMLBundle (not a string) in Vercel's
serverless environment, and Bun.file() has no filesystem access there.
Copy the HTML content into api/test-api-content.ts as an escaped string
export, then import it in api/relay.ts — Bun bundler inlines it.

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

api/relay.ts

@@ -29,10 +29,7 @@ import { createRateLimiter } from "../src/middleware/rate-limiter";
 import { logRelayEvent } from "../src/middleware/logger";
 import { handleChatCompletion, listModels } from "../src/lib/ai-proxy";
 import { handleAnthropicMessages } from "../src/lib/anthropic-proxy";
-
-// Read test-api.html at module scope so Bun's bundler inlines the content
-// at build time (Vercel serverless has no filesystem at runtime).
-const testApiHtmlRaw = await Bun.file("public/test-api.html").text();
+import testApiHtmlRaw from "./test-api-content";
 
 // ─── Configuration ──────────────────────────────────────────────────────────────