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/antigravity.js

@@ -1,4 +1,5 @@
 import { platform, arch } from "os";
+import { ANTIGRAVITY_OAUTH_CLIENT } from "../shared.js";
 
 export default {
   id: "antigravity",
@@ -10,8 +11,7 @@ export default {
     ],
     format: "antigravity",
     headers: { "User-Agent": `antigravity/1.107.0 ${platform()}/${arch()}` },
-    clientId: "1071006060591-tmhssin2h21lcre235vtolojh4g403ep.apps.googleusercontent.com",
-    clientSecret: "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"
+    ...ANTIGRAVITY_OAUTH_CLIENT
   },
   models: [
     { id: "gemini-3-flash-agent", name: "Gemini 3.5 Flash (High)" },