feat(ai-client): multi-provider compat — native Anthropic/Google + URL tolerance

- TEXT/VISION: add native Anthropic & Google Gemini paths via Vercel AI SDK,
  selectable through TEXT_PROVIDER / VISION_PROVIDER (default openai_compatible)
- IMAGE: expand to openai (gpt-image) / google (Nano Banana) via AI SDK
  alongside the existing Runware task-array and OpenAI-compatible REST paths
- normalizeBaseUrl: tolerate URLs with/without /v1 (or /chat/completions);
  append the per-protocol version segment only for bare hosts
- config: readProvider() reads *_PROVIDER; types: ProviderProtocol + provider?
- deps: @ai-sdk/anthropic, @ai-sdk/google; docs in .env.example + README

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

package.json

@@ -20,6 +20,10 @@
     "deploy:cf": "opennextjs-cloudflare deploy"
   },
   "dependencies": {
+    "@ai-sdk/anthropic": "^3.0.81",
+    "@ai-sdk/google": "^3.0.80",
+    "@ai-sdk/openai": "^3.0.67",
+    "ai": "^6.0.196",
     "jsonrepair": "^3.14.0",
     "next": "^16.0.0",
     "react": "^19.0.0",