Add Google Vertex AI provider (#1163)

# Summary

* Adds first-class **Google Vertex AI provider** using
`@ai-sdk/google-vertex`.
* Supports **Gemini 2.5** models and partner **MaaS (Model Garden)**
models via full publisher IDs.
* New **Vertex-specific settings UI** for Project, Location, and Service
Account JSON.
* Implements a **“thinking” toggle** for Gemini 2.5 Flash

  * Pro: always on
  * Flash: toggleable
  * Flash Lite: none
* Fixes *“AI not found”* for Vertex built-ins by mapping to
`publishers/google` paths.
* Hardens **cross-platform file ops** and ensures all tests pass.

---

# What’s New

### Vertex AI Provider

* Uses `@ai-sdk/google-vertex` with `googleAuthOptions.credentials` from
pasted Service Account JSON.
* Configurable **project** and **location**.
* Base URL → `/projects/{project}/locations/{location}`

  * Built-ins: `publishers/google/models/<id>`
  * Partner MaaS: `publishers/<partner>/models/...`

### Built-in Vertex Models

* `gemini-2.5-pro`
* `gemini-2.5-flash`
* `gemini-2.5-flash-lite`

### Thinking Behavior

* Vertex + Google marked as thinking-capable.
* Pro: always thinking
* Flash: toggle in UI
* Flash Lite: none

### Vertex Settings UI

* New **Google Vertex AI panel** for Project ID, Location, Service
Account JSON.
* Keys encrypted like other secrets.

---

# Fixes

* **Model resolution:** built-ins auto-map to
`publishers/google/models/<id>`.
* **Partner MaaS support:** full publisher IDs work directly (e.g.
DeepSeek).
* **Cross-platform paths:** normalize file ops with `toPosixPath`,
preserve `safeJoin` semantics.

---

# Why This Is Better

* Users can select **Vertex alongside other providers**.
* **More models** available through Model Garden.
* **Dedicated setup UI** reduces misconfig.
* **Thinking toggle** gives control over cost vs. reasoning depth.

---

# Files Changed

* **Provider & Models**: `language_model_helpers.ts`,
`get_model_client.ts`
* **Streaming**: `chat_stream_handlers.ts`
* **Schemas & Encryption**: `schemas.ts`, `settings.ts`
* **Settings UI**: `VertexConfiguration.tsx`, `ApiKeyConfiguration.tsx`
* **Models UI**: `ModelsSection.tsx` (Flash toggle)
* **Setup Detection**: `useLanguageModelProviders.ts`
* **Path Utils**: `path_utils.ts`, `response_processor.ts`
* **Deps**: `package.json` → `@ai-sdk/google-vertex@3.0.16`

---

# Tests & Validation

* **TypeScript**: `npm run ts` → ✅
* **Lint**: `npm run lint` → ✅
* **Unit tests**: `npm test` → ✅ 231 passed, 0 failed

---

# Migration / Notes

* No breaking changes.
* For Vertex usage:

  * Ensure Vertex AI API is enabled.
  * Service Account needs `roles/aiplatform.user`.
  * Region must support model (e.g. `us-central1`).
* Thinking toggle currently affects **only** Gemini 2.5 Flash.

---

# Manual QA

1. Configure Vertex with Project/Location/Service Account JSON.
2. Test built-ins:

   * `gemini-2.5-pro`
   * `gemini-2.5-flash` (toggle on/off)
   * `gemini-2.5-flash-lite`
3. Test MaaS partner model (e.g., DeepSeek) via full publisher ID.
4. Verify other providers remain unaffected.
    
<!-- This is an auto-generated description by cubic. -->
---

## Summary by cubic
Adds a first-class Google Vertex AI provider with Gemini 2.5 models, a
Vertex settings panel, and a “thinking” toggle for Gemini 2.5 Flash.
Also fixes model resolution for Vertex and hardens cross-platform file
operations.

- **New Features**
- Vertex AI provider via @ai-sdk/google-vertex with project, location,
and service account JSON.
- Built-in models: gemini-2.5-pro, gemini-2.5-flash,
gemini-2.5-flash-lite.
- “Thinking” support: Pro always on; Flash toggle in Models UI; Flash
Lite none.
- MaaS partners supported via full publisher paths (e.g.,
publishers/<partner>/models/...).
  - Vertex settings UI with encrypted service account key storage.

- **Bug Fixes**
  - Built-in Vertex models auto-map to publishers/google/models/<id>.
  - Consistent file ops across platforms using toPosixPath.
- Vertex readiness detection requires project/location/service account
JSON.
- Streaming “thinking” behavior respects Vertex Flash toggle and Pro
always-on.

<!-- End of auto-generated description by cubic. -->

---------

Co-authored-by: Md Rakibul Islam Rocky <mdrirocky08@gmail.com>
Co-authored-by: graphite-app[bot] <96075541+graphite-app[bot]@users.noreply.github.com>
Co-authored-by: Will Chen <willchen90@gmail.com>

package.json

@@ -88,6 +88,7 @@
     "@ai-sdk/anthropic": "^2.0.4",
     "@ai-sdk/azure": "^2.0.17",
     "@ai-sdk/google": "^2.0.6",
+    "@ai-sdk/google-vertex": "3.0.16",
     "@ai-sdk/openai": "2.0.15",
     "@ai-sdk/openai-compatible": "^1.0.8",
     "@ai-sdk/provider-utils": "^3.0.3",