Browser agent and web search tools not working

### What happened?

[ACTION REQUIRED] 📎 PLEASE ATTACH THE EXPORTED CHAT HISTORY JSON FILE TO THIS ISSUE IF YOU FEEL COMFORTABLE SHARING IT.

### What did you expect to happen?

The agent's web-enabled tools (google_web_search and web_fetch) are consistently failing with an INVALID_MODEL error. This prevents the agent from
performing any tasks that require accessing external web resources, such as web searches or fetching content from URLs.

The expected behavior is that these tools should connect to the internet and return information. Instead, they return a system error pointing to an issue
with the gemini-3-flash-preview model.

### Client information

* **CLI Version:** 0.50.0
* **Git Commit:** 8d9733d53
* **Session ID:** 6e8ddcff-8cf5-41c4-8372-4124210e4867
* **Operating System:** win32 v24.16.0
* **Sandbox Environment:** no sandbox
* **Model Version:** gemini-2.5-pro
* **Auth Type:** gemini-api-key
* **Memory Usage:** 329.9 MB
* **Terminal Name:** Unknown
* **Terminal Background:** #0c0c0c
* **Kitty Keyboard Protocol:** Unsupported

### Login information

Logged in using API key

### Anything else we need to know?

_No response_

Start by reproducing failures in google_web_search and web_fetch with CLI version 0.50.0, commit 8d9733d53, Gemini API-key authentication, and the listed model versions. Trace the INVALID_MODEL response associated with gemini-3-flash-preview; done means both web-enabled tools connect and return search or fetched content instead of the system error.

Written by the indexing model from the issue text.