[Enhancement] Refactor "Test Connection" to Support Multiple Provider Types (Azure, Gemini, etc.)

[HexdoAI]

Parent Epic: #56
Related: #61 (Azure), #62, #63

🎯 Goal

To refactor the existing "Test Connection" button so it can dynamically validate credentials for all supported provider_type (OpenAI, Azure, Gemini, Open Router), not just the default OpenAI format.

🐛 Current Problem

The "Test Connection" button already exists, but its backend logic is hard-coded to send a test request only in the format expected by the standard OpenAI API.

This means:

• When a user correctly configures an Azure provider and clicks "Test," the test fails because the backend doesn't send the api-version or use the correct endpoint structure.
• This will also fail for Gemini and Open Router as they have unique API formats.

⚙️ Backend Requirements

The "Test Connection" API endpoint (e.g., /api/model/test-connection) must be refactored:

1. Read Provider Type: The endpoint must receive the entire form configuration from the frontend, including the provider_type (OpenAI, Azure, Gemini, etc.).
2. Dynamic Dispatch: The backend must use the provider_type to determine which test logic to run. A switch statement or factory pattern is recommended.
   • Case 'OpenAI': Use the existing validation logic.
   • Case 'Azure': Use the Azure-specific logic (e.g., attempt a simple chat completion using the Endpoint, Deployment ID, API Key, and API Version).
   • Case 'Gemini': Use Gemini-specific logic (e.g., call generateContent with the API Key).
   • Case 'OpenRouter': Use OpenRouter-specific logic (e.g., make a request with the Authorization and HTTP-Referer headers).
3. Unified Response: The endpoint must return a standardized JSON response, regardless of the provider tested:
   • { success: true }
   • { success: false, error: "Connection failed: [Specific error from the provider]" }

💻 UI Requirements (Frontend)

• No new UI elements are needed (the button already exists).
• The frontend "Test Connection" button's click handler must be updated to send the entire form state (including the selected provider_type) to the backend test endpoint.

✅ Acceptance Criteria

• The "Test Connection" button continues to work correctly for standard OpenAI configurations.
• The "Test Connection" button now works correctly for a valid Azure configuration.
• The "Test Connection" button now works correctly for a valid Gemini configuration.
• The "Test Connection" button now works correctly for a valid Open Router configuration.
• If a test fails for any provider, a clear error message from that provider is shown to the user.