fix(agent): correctly strip <final> tags from reasoning providers

- Added src/utils/provider-utils.ts to track reasoning provider logic
- Updated isReasoningTagProvider to loosely match 'google-antigravity' (fixes sub-models)
- Enabled enforceFinalTag in reply.ts when provider matches
- Verified <final> tag stripping logic in pi-embedded-subscribe.ts
- Updated pi-embedded-runner to use consistent provider check for prompt hints

src/agents/pi-embedded-helpers.ts

@@ -183,7 +183,11 @@ export async function sanitizeSessionMessagesImages(
 const GOOGLE_TURN_ORDER_BOOTSTRAP_TEXT = "(session bootstrap)";
 
 export function isGoogleModelApi(api?: string | null): boolean {
-  return api === "google-gemini-cli" || api === "google-generative-ai";
+  return (
+    api === "google-gemini-cli" ||
+    api === "google-generative-ai" ||
+    api === "google-antigravity"
+  );
 }
 
 export function sanitizeGoogleTurnOrdering(