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

src/shared/constants/providers.js

@@ -51,6 +51,8 @@ export const APIKEY_PROVIDERS = {
   chutes: { id: "chutes", alias: "ch", name: "Chutes AI", icon: "water_drop", color: "#ffffffff", textIcon: "CH", website: "https://chutes.ai" },
   ollama: { id: "ollama", alias: "ollama", name: "Ollama Cloud", icon: "cloud", color: "#ffffffff", textIcon: "OL", website: "https://ollama.com" },
   "ollama-local": { id: "ollama-local", alias: "ollama-local", name: "Ollama Local", icon: "cloud", color: "#ffffffff", textIcon: "OL", website: "https://ollama.com" },
+  vertex: { id: "vertex", alias: "vx", name: "Vertex AI", icon: "cloud", color: "#4285F4", textIcon: "VX", website: "https://cloud.google.com/vertex-ai" },
+  "vertex-partner": { id: "vertex-partner", alias: "vxp", name: "Vertex Partner", icon: "cloud", color: "#34A853", textIcon: "VP", website: "https://cloud.google.com/vertex-ai/generative-ai/docs/partner-models/use-partner-models" },
 };
 
 export const OPENAI_COMPATIBLE_PREFIX = "openai-compatible-";