fix(usage): read Gemini usageMetadata out of the antigravity response envelope

Antigravity and gemini-cli wrap their payload in { response: {...} }.
extractUsageFromResponse only tested top-level usageMetadata, so every
non-streaming antigravity request logged zero usage (IN 0 | OUT 0) and
zeroed rows in the usage dashboard. Read the envelope the same way
usageTracking.js and nonStreamingHandler.js already do; top-level
metadata keeps priority and the OpenAI/Claude branches are untouched.

Fixes #3260

open-sse/handlers/chatCore/requestDetail.js

@@ -44,13 +44,14 @@ export function extractUsageFromResponse(responseBody) {
     };
   }
 
-  // Gemini format
-  if (responseBody.usageMetadata) {
+  // Gemini format. Antigravity / gemini-cli wrap the payload in { response: {...} }.
+  const usageMetadata = responseBody.usageMetadata || responseBody.response?.usageMetadata;
+  if (usageMetadata) {
     return {
-      prompt_tokens: responseBody.usageMetadata.promptTokenCount || 0,
-      completion_tokens: responseBody.usageMetadata.candidatesTokenCount || 0,
-      cached_tokens: responseBody.usageMetadata.cachedContentTokenCount || 0,
-      reasoning_tokens: responseBody.usageMetadata.thoughtsTokenCount || 0
+      prompt_tokens: usageMetadata.promptTokenCount || 0,
+      completion_tokens: usageMetadata.candidatesTokenCount || 0,
+      cached_tokens: usageMetadata.cachedContentTokenCount || 0,
+      reasoning_tokens: usageMetadata.thoughtsTokenCount || 0
     };
   }