[Bug] Missing Content-Type header & upcoming DeepSeek model rename

[GoldenHoe]

Expected Behavior

1. After clicking the "Test" button, the DeepSeek API should be successfully connected and return a normal response.
2. The model selection dropdown should contain the latest DeepSeek model names to avoid future unavailability.

Problem Description

Issue 1: HTTP request header missing Content-Type

After configuring the DeepSeek API and clicking "Test", a 415 error is returned:

Status code: 415
Error body:
Expected request with Content-Type: application/json

The cause is that the plugin does not add Content-Type: application/json to the request headers when initiating the request.

Issue 2: Model name to be deprecated soon

DeepSeek official policy update: The currently used deepseek-chat model name will be officially deprecated in July 2026, and will be changed to:

• deepseek-v4-flash
• deepseek-v4-pro

(For details, refer to: https://api-docs.deepseek.com)

If the plugin does not update the model list, API calls will fail after July.

Steps to Reproduce

1. Open the AI Writing Assistant configuration page.
2. Select DeepSeek as the third-party model.
3. Fill in a valid API Key, with Base URL as https://api.deepseek.com.
4. Select the deepseek-chat model.
5. Click the "Test" button → a 415 error appears.
6. Ignore the error and continue using it; after July, it will fail again due to the model name deprecation.

Requested Fixes

1. Explicitly add Content-Type: application/json to the request headers.
2. Update the model dropdown list: replace or extend deepseek-chat with deepseek-v4-flash and deepseek-v4-pro.

Environment where the issue occurred

• Device: Xiaomi 220233L2C(Redmi 10A)
• OS: Android 13(Pixel Experience 13)
• Kori version: 1.1.8

Screenshots to reproduce the issue

Thank you for taking the time to look into this issue. Please let me know if you need any more information or want me to test any patches.