fix(core): keep vertex settings serializable

[kitlangton]

Why

The Google Vertex catalog transform stored a runtime fetch function in provider settings. Catalog model projection JSON-merges provider and model settings, so reading a compatible Vertex model could fail schema validation before execution.

What Changes

The catalog transform now retains only serializable project, location, and endpoint settings. Google authentication fetch injection remains in the existing AI SDK runtime hook.

The regression adds a model to the existing compatible-Vertex catalog case and reads its projected settings through the real Catalog service.

Scope

This fixes the catalog/settings boundary only. It does not change native provider selection or claim to add ADC authentication to the native OpenAI-compatible route.

Verification

cd packages/core
bun run test test/plugin/provider-google-vertex.test.ts
bun typecheck

cd ../..
bunx prettier --check packages/core/src/plugin/provider/google-vertex.ts packages/core/test/plugin/provider-google-vertex.test.ts
bunx oxlint packages/core/src/plugin/provider/google-vertex.ts packages/core/test/plugin/provider-google-vertex.test.ts
git diff --check
git push -u origin vertex-settings # repository hook: full workspace typecheck

Focused result: 12 tests passed. Core typecheck, formatting, diff validation, and the full 39-package push-hook typecheck passed. Oxlint completed with only pre-existing warnings in the test file.