feat(ai-gemini): upgrade @google/genai to v2 + migrate Interactions API

.changeset/gemini-genai-v2-upgrade.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/ai-gemini': minor
+---
+
+Upgrade `@google/genai` to v2 and migrate the experimental text-interactions adapter to the SDK 2.x step-based streaming API (`step.start` / `step.delta` / `step.stop`), replacing the prior `content.*` events. Streamed function-call arguments are now accumulated from `arguments_delta` fragments and parsed leniently with `partial-json`, so an incomplete or truncated buffer keeps the last good arguments instead of resetting them and no longer logs a parse error per fragment. Exported built-in-tool CUSTOM event payloads now carry `Interactions.*Step` values, and structured output uses the polymorphic `response_format` request shape.

packages/ai-gemini/package.json

@@ -64,8 +64,9 @@
     "text-to-speech"
   ],
   "dependencies": {
-    "@google/genai": "^1.43.0",
-    "@tanstack/ai-utils": "workspace:*"
+    "@google/genai": "^2.8.0",
+    "@tanstack/ai-utils": "workspace:*",
+    "partial-json": "^0.1.7"
   },
   "peerDependencies": {
     "@tanstack/ai": "workspace:^"