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>

src/lib/oauth/constants/oauth.js

@@ -2,6 +2,7 @@
  * OAuth Configuration Constants
  */
 import { platform, arch } from "os";
+import { ANTIGRAVITY_OAUTH_CLIENT, GOOGLE_OAUTH_CLIENT } from "open-sse/providers/shared.js";
 
 /**
  * Get the platform enum value based on the current OS.
@@ -42,8 +43,7 @@ export const CODEX_CONFIG = {
 
 // Gemini (Google) OAuth Configuration (Standard OAuth2)
 export const GEMINI_CONFIG = {
-  clientId: "681255809395-oo8ft2oprdrnp9e3aqf6av3hmdib135j.apps.googleusercontent.com",
-  clientSecret: "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",
+  ...GOOGLE_OAUTH_CLIENT,
   authorizeUrl: "https://accounts.google.com/o/oauth2/v2/auth",
   tokenUrl: "https://oauth2.googleapis.com/token",
   userInfoUrl: "https://www.googleapis.com/oauth2/v1/userinfo",
@@ -94,8 +94,7 @@ export const IFLOW_CONFIG = {
 
 // Antigravity OAuth Configuration (Standard OAuth2 with Google)
 export const ANTIGRAVITY_CONFIG = {
-  clientId: "1071006060591-tmhssin2h21lcre235vtolojh4g403ep.apps.googleusercontent.com",
-  clientSecret: "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",
+  ...ANTIGRAVITY_OAUTH_CLIENT,
   authorizeUrl: "https://accounts.google.com/o/oauth2/v2/auth",
   tokenUrl: "https://oauth2.googleapis.com/token",
   userInfoUrl: "https://www.googleapis.com/oauth2/v1/userinfo",