Title: Support JSON arrays in custom model body parameters for OpenAI Responses API providers

Description: When using a custom model with API Type = "OpenAI Responses API (New)" against xAI's /v1/responses endpoint, TypingMind currently appears to serialize custom body parameters in a way that prevents arrays from being passed correctly.

Use case: xAI model: grok-4.20-multi-agent-beta-0309 Endpoint: https://api.x.ai/v1/responses

This model supports built-in tools such as: [ {"type":"web_search"}, {"type":"x_search"} ]

The request must send tools as a real JSON array.

Problem: In the custom model UI, bodyRows do not seem to allow passing tools as an actual JSON array. When trying to pass: [{"type":"web_search"},{"type":"x_search"}]

TypingMind sends it as a string instead of a sequence/array.

Observed server error: Failed to deserialize the JSON body into the target type: tools: invalid type: string "[{"type":"web_search"},{"type":"x_search"}]", expected a sequence

Impact:

• The custom model itself works

• Responses API works

• But provider-native built-in tools cannot be used

• This blocks full compatibility with xAI multi-agent / responses-based models

Requested improvement: Please allow true JSON arrays in custom model body parameters for Responses API models, or provide an advanced raw JSON body editor for custom models.

This would enable compatibility with providers/models that require arrays in request bodies, such as xAI Responses API tools.

This is not just a new feature request, but also a compatibility issue: TypingMind can already call the model successfully, but cannot pass provider-native tool arrays correctly through the custom model body UI.