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/tools.ts

@@ -117,11 +117,13 @@ export function normalizeToolsForGemini(
 
       functionDeclarations.push(declaration)
     } else {
-      // Log warning for unsupported tool types
-      console.warn(
-        `[antigravity-tools] Unsupported tool type: "${tool.type}". ` +
-          `Only "function" type tools are supported for Gemini. Tool will be skipped.`
-      )
+      // Log warning for unsupported tool types (debug only)
+      if (process.env.ANTIGRAVITY_DEBUG === "1") {
+        console.warn(
+          `[antigravity-tools] Unsupported tool type: "${tool.type}". ` +
+            `Only "function" type tools are supported for Gemini. Tool will be skipped.`
+        )
+      }
     }
   }