feat: adapt cloudflare worker ai (#14732)

#### PR Dependency Tree


* **PR #14732** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Cloudflare Workers AI added as a Copilot provider (configurable in
admin settings).
  * Gemini 3.1 Flash Lite Preview model made available.

* **Behavior Changes**
* Reranking now uses a different default model identifier (affects
relevancy scores).

* **Tests**
* Rerank tests adjusted to focus on the updated model and expected
results.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/admin/src/config.json

@@ -326,6 +326,10 @@
       "desc": "The config for the openai provider.",
       "link": "https://github.com/openai/openai-node"
     },
+    "providers.cloudflareWorkersAi": {
+      "type": "Object",
+      "desc": "The config for the Cloudflare Workers AI provider."
+    },
     "providers.fal": {
       "type": "Object",
       "desc": "The config for the fal provider."