fix(antigravity-auth): address Oracle feedback - custom credentials, logging, docs

- Fix custom credentials to actually work in OAuth/refresh flows
  - oauth.ts: Add clientId/clientSecret parameters to buildAuthURL(), exchangeCode()
  - token.ts: Add clientId/clientSecret parameters to refreshAccessToken()
  - fetch.ts: Pass credentials to oauth/token functions
  - plugin.ts: Use closure cache for credentials, pass to all flows

- Unify console.* logging policy with ANTIGRAVITY_DEBUG guards
  - constants.ts: Document logging policy
  - tools.ts: Guard console.warn with ANTIGRAVITY_DEBUG
  - plugin.ts: Guard 4 console.error with ANTIGRAVITY_DEBUG

- Add explicit init.body type handling
  - fetch.ts: Check body type, pass-through non-string bodies
  - fetch.ts: Document body type assumption

- Document SSE buffering behavior
  - response.ts: Add warning that current implementation buffers
  - response.ts: Add TODO for future ReadableStream enhancement

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/auth/antigravity/token.ts

@@ -36,16 +36,20 @@ export function isTokenExpired(tokens: AntigravityTokens): boolean {
  * Exchanges the refresh token for a new access token via Google's OAuth endpoint.
  *
  * @param refreshToken - The refresh token to use
+ * @param clientId - Optional custom client ID (defaults to ANTIGRAVITY_CLIENT_ID)
+ * @param clientSecret - Optional custom client secret (defaults to ANTIGRAVITY_CLIENT_SECRET)
  * @returns Token exchange result with new access token, or throws on error
  */
 export async function refreshAccessToken(
-  refreshToken: string
+  refreshToken: string,
+  clientId: string = ANTIGRAVITY_CLIENT_ID,
+  clientSecret: string = ANTIGRAVITY_CLIENT_SECRET
 ): Promise<AntigravityTokenExchangeResult> {
   const params = new URLSearchParams({
     grant_type: "refresh_token",
     refresh_token: refreshToken,
-    client_id: ANTIGRAVITY_CLIENT_ID,
-    client_secret: ANTIGRAVITY_CLIENT_SECRET,
+    client_id: clientId,
+    client_secret: clientSecret,
   })
 
   const response = await fetch(GOOGLE_TOKEN_URL, {