Agents window allows chat without an agent selected

[Micheletto]

What happened?

The Agents window will allow a conversation to start even when an agent is not selected. This results in an error: ```
Something went wrong. Here's the specific error message we encountered: An error occurred while processing your request. Please contact the Admin.

For a better user experience this ideally wouldn't be possible. Alternatively, an error message that indicates the problem would be more useful to users.

### Version Information

Running: 
librechat                                         latest                                     93a2f086271e   11 days ago     892MB


### Steps to Reproduce

1. Create a new user.
2. Go to the Agents (may be default), don't select an agent.
3. Enter any prompt.

![Image](https://github.com/user-attachments/assets/4a6be1e4-dfd1-4e7e-b163-cea85a4cede8)

### What browsers are you seeing the problem on?

Firefox

### Relevant log output

```shell

Screenshots

No response

Code of Conduct

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[danny-avila]

We'll add this soon, thanks