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/services/provider.js

@@ -298,6 +298,12 @@ export function buildProviderHeaders(provider, credentials, stream = true, body
         // Claude-compatible API providers use x-api-key
         headers["x-api-key"] = credentials.apiKey;
         break;
+
+      case "vertex":
+      case "vertex-partner":
+        // Vertex uses async token minting — headers are set by VertexExecutor._buildHeadersAsync()
+        // Do NOT set Authorization here; it would leak the raw SA JSON as Bearer token
+        break;
   
       default:
         headers["Authorization"] = `Bearer ${credentials.apiKey || credentials.accessToken}`;