feat: Add Google Cloud Vertex AI provider support (vertex, vertex-partner)

Co-authored-by: Quan <quanle96@outlook.com>
PR: https://github.com/decolua/9router/pull/298

Thanks to @kwanLeeFrmVi for the original implementation. Here is a summary
of changes made during review integration:

- Replaced google-auth-library with jose (already a project dependency)
  for SA JSON -> OAuth2 Bearer token minting (RS256 JWT assertion flow)
- Moved auth logic (parseSaJson, refreshVertexToken, token cache) from
  executor into open-sse/services/tokenRefresh.js to match project pattern
- Fixed executor to use proxyAwareFetch instead of raw fetch (proxy support)
- Simplified buildUrl: use global aiplatform.googleapis.com endpoint for
  both vertex (Gemini) and vertex-partner; removed region/modelFamily fields
- Added auto-detection of GCP project_id from raw API key via probe request
  (vertex-partner only, cached per key)
- Added vertex/vertex-partner cases to /api/providers/validate/route.js
- Updated model lists based on live testing:
  - vertex: gemini-3.1-pro-preview, gemini-3.1-flash-lite-preview,
    gemini-3-flash-preview, gemini-2.5-flash (removed gemini-2.5-pro: 404)
  - vertex-partner: deepseek-v3.2, qwen3-next-80b (instruct+thinking),
    glm-5 (removed Mistral/Llama: not enabled in test project)
  - gemini provider: added gemini-3.1-pro-preview, gemini-3.1-flash-lite-preview
- Removed bun.lock (project uses npm/package-lock.json)
- Removed region and modelFamily UI fields (global endpoint, auto-detect)
- Kiro token auto-refresh on AccessDeniedException (from commit 2)

Made-with: Cursor

open-sse/config/providers.js

@@ -298,4 +298,16 @@ export const PROVIDERS = {
     baseUrl: "http://localhost:11434/api/chat",
     format: "ollama"
   },
+  // Vertex AI - Gemini models via Service Account JSON
+  // baseUrl is not used; VertexExecutor.buildUrl() constructs it dynamically
+  vertex: {
+    baseUrl: "https://aiplatform.googleapis.com",
+    format: "gemini"
+  },
+  // Vertex AI - Partner models (Claude, Llama, Mistral, GLM) via SA JSON
+  // Uses OpenAI-compatible global endpoint (or rawPredict for Anthropic)
+  "vertex-partner": {
+    baseUrl: "https://aiplatform.googleapis.com",
+    format: "openai"
+  },
 };