feat: add AnthropicVertexClient for claude-* models

- @anthropic-ai/vertex-sdk: proper Anthropic Messages API on Vertex
- AnthropicVertexClient: converts OAI message format ↔ Anthropic format,
  handles tool_use blocks, retries 429/503 with backoff
- createLLM: routes anthropic/* and claude-* models through new client
- Tier B/C default: claude-sonnet-4-6 via us-east5 Vertex endpoint
- /generate endpoint: accepts region param for regional endpoint testing

Made-with: Cursor

package.json

@@ -10,6 +10,8 @@
     "test": "ts-node src/test.ts"
   },
   "dependencies": {
+    "@anthropic-ai/sdk": "^0.78.0",
+    "@anthropic-ai/vertex-sdk": "^0.14.4",
     "@google/genai": "^1.0.0",
     "cors": "^2.8.5",
     "express": "^4.19.2",