chore(deps): update google-genai requirement in /server/ai-gateway

Updates the requirements on [google-genai](https://github.com/googleapis/python-genai) to permit the latest version.
- [Release notes](https://github.com/googleapis/python-genai/releases)
- [Changelog](https://github.com/googleapis/python-genai/blob/main/CHANGELOG.md)
- [Commits](googleapis/python-genai@v1.0.0...v2.18.0)

---
updated-dependencies:
- dependency-name: google-genai
  dependency-version: 2.16.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

server/ai-gateway/requirements.txt

@@ -12,5 +12,5 @@ python-jose[cryptography]==3.5.0
 httpx==0.28.1
 python-dotenv==1.2.2
 python-json-logger==3.3.0
-google-genai>=1.0.0
+google-genai>=2.18.0
 python-multipart==0.0.32