Support OpenAI /v1/responses API and configurable model-specific parameters (e.g. reasoning_effort)

When using newer OpenAI models in PowerShell Universal, the current implementation appears to be limited to the legacy /v1/chat/completions endpoint.
This creates compatibility issues with newer reasoning-capable models such as gpt-6.1-sol. These models have different API requirements and may require the newer /v1/responses endpoint for function/tool calling scenarios.
For example, when using tools/functions with gpt-6.1-sol, OpenAI returns:

HTTP 400 (invalid_request_error)

Function tools with reasoning_effort are not supported for gpt-6.1-sol in /v1/chat/completions.
To use function tools, use /v1/responses or set reasoning_effort to 'none'.

At the moment there appears to be no way in PowerShell Universal to:

1. Select the OpenAI API endpoint (/v1/chat/completions vs /v1/responses).
2. Configure model-specific parameters such as:
   • reasoning_effort
   • reasoning settings in general
   • future model-specific options introduced by OpenAI.

As a result, users are unable to use newer OpenAI models with tool/function calling even though the models themselves support the required functionality through the Responses API.

Requested Enhancements
Feature 1: Responses API Support
Add support for OpenAI's /v1/responses endpoint and allow selecting the API type in the provider configuration.
Example:

New-PSUAIProvider -ApiType Responses

or

New-PSUAIProvider -EndpointType Responses

Feature 2: Custom Model Parameters
Allow arbitrary model parameters to be passed through to OpenAI.
Example:

New-PSUAIAgent `
    -Model "gpt-6.1-sol" `
    -ModelParameter @{
        reasoning_effort = "none"
    }

or a similar mechanism on the provider level.

Benefit
This would allow PowerShell Universal users to adopt newer OpenAI models immediately and avoid compatibility issues caused by hard-coded assumptions around the Chat Completions API and fixed model parameters.