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/__tests__/mocks/copilot.mock.ts

@@ -33,7 +33,7 @@ export class MockCopilotProvider extends OpenAIProvider {
       id: 'test-image',
       capabilities: [
         {
-          input: [ModelInputType.Text],
+          input: [ModelInputType.Text, ModelInputType.Image],
           output: [ModelOutputType.Image],
           defaultForOutputType: true,
         },