build(deps): bump @openai/agents from 0.3.8 to 0.4.0

Bumps [@openai/agents](https://github.com/openai/openai-agents-js) from 0.3.8 to 0.4.0.
- [Release notes](https://github.com/openai/openai-agents-js/releases)
- [Commits](openai/openai-agents-js@v0.3.8...v0.4.0)

---
updated-dependencies:
- dependency-name: "@openai/agents"
  dependency-version: 0.4.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

packages/core/package.json

@@ -40,7 +40,7 @@
   "license": "MIT",
   "type": "module",
   "dependencies": {
-    "@openai/agents": "^0.3.8",
+    "@openai/agents": "^0.4.0",
     "@openai/agents-extensions": "^0.3.8",
     "lru-cache": "^11.2.4",
     "zod": "^4.1.13"