Add "Test Connection" Button to Repository Creation to Validate Connectivity and Data Retrieval

### Current Behavior

When creating a new repository in Dependency-Track (e.g., Rust/Cargo), users must provide:

- Identifier
- URL
- Repository Type
- Optional authentication settings

Currently, there is no way to validate the repository configuration before saving it.

If the URL is incorrect, authentication fails, the repository type does not match the endpoint, or the server is unreachable, the failure is only discovered later during background processing or scheduled jobs.

This results in:
- Delayed feedback
- Additional troubleshooting effort
- Creation of misconfigured repository entries

### Proposed Behavior

Add a **"Test Connection"** button to the "Create Repository" dialog.

The button should:

1. Send the current form configuration (without persisting it) to the backend.
2. Perform a server-side validation that:
- The repository URL is reachable
- Authentication (if enabled) is valid
- The response matches the expected format for the selected repository type
3. Return a clear result to the UI

### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [x] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this enhancement was already requested