refactor(open-sse): dedupe Google OAuth client credentials (#4)

clientId/clientSecret của antigravity + gemini bị lặp 3 nơi
(registry, usage.js, src/lib/oauth). Gom vào shared.js
(ANTIGRAVITY_OAUTH_CLIENT, GOOGLE_OAUTH_CLIENT), các file spread vào.
Byte-for-byte: PROVIDERS/alias/oauth-url equal, golden 142 pass.
Thêm test guard nội dung + alias resolution.

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/providers/registry/gemini-cli.js

@@ -1,11 +1,12 @@
+import { GOOGLE_OAUTH_CLIENT } from "../shared.js";
+
 export default {
   "id": "gemini-cli",
   "alias": "gc",
   "transport": {
     "baseUrl": "https://cloudcode-pa.googleapis.com/v1internal",
     "format": "gemini-cli",
-    "clientId": "681255809395-oo8ft2oprdrnp9e3aqf6av3hmdib135j.apps.googleusercontent.com",
-    "clientSecret": "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"
+    ...GOOGLE_OAUTH_CLIENT
   },
   "models": [
     {