feat(server): migrate copilot to native (#14620)

#### PR Dependency Tree


* **PR #14620** 👈

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**
* Native LLM workflows: structured outputs, embeddings, and reranking
plus richer multimodal attachments (images, audio, files) and improved
remote-attachment inlining.

* **Refactor**
* Tooling API unified behind a local tool-definition helper;
provider/adapters reorganized to route through native dispatch paths.

* **Chores**
* Dependency updates, removed legacy Google SDK integrations, and
increased front memory allocation.

* **Tests**
* Expanded end-to-end and streaming tests exercising native provider
flows, attachments, and rerank/structured scenarios.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/copilot/config.ts

@@ -81,7 +81,7 @@ export type CopilotProviderProfile = CopilotProviderProfileCommon &
   }[CopilotProviderType];
 
 export type CopilotProviderDefaults = Partial<
-  Record<ModelOutputType, string>
+  Record<Exclude<ModelOutputType, ModelOutputType.Rerank>, string>
 > & {
   fallback?: string;
 };
@@ -184,6 +184,7 @@ const CopilotProviderDefaultsShape = z.object({
   [ModelOutputType.Object]: z.string().optional(),
   [ModelOutputType.Embedding]: z.string().optional(),
   [ModelOutputType.Image]: z.string().optional(),
+  [ModelOutputType.Rerank]: z.string().optional(),
   [ModelOutputType.Structured]: z.string().optional(),
   fallback: z.string().optional(),
 });