chatOptions.baseSystemMessage option in the json config to being able to override the default system prompt

[ferenci84]

Description

Previously it was impossible to override the default system prompt. Yaml config now has chatOptions.baseSystemMessage that can do that, but not available from json config.

Now added chatOptions.baseSystemMessage config option in the json config similar to the yaml config to being able to override the system message.

Issue: #5256

Checklist

• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Testing instructions

You should be able to override the default system prompt with any of the following model config:

    {
      "title": "Anthropic Claude 3.7 Sonnet",
      "model": "claude-3-7-sonnet-latest",
      "provider": "anthropic",
      "apiKey": "YOUR_API_KEY",
      "systemMessage": "Your own system message",
      "chatOptions": {
        "baseSystemMessage": ""
      },

    {
      "title": "Anthropic Claude 3.7 Sonnet",
      "model": "claude-3-7-sonnet-latest",
      "provider": "anthropic",
      "apiKey": "YOUR_API_KEY",
      "chatOptions": {
        "baseSystemMessage": "Your own system message"
      },

Just setting systemMessage only should keep the legacy behaviour: systemMessage is appended to the default prompt with two newlines.

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[RomneyDa]

@ferenci84 JSON model systemMessage is already converted to the base chat system message, so this functionality is duplicate. Are you finding that this isn't working?