## Purpose

Define the shared data models and API contract for system prompt and model settings — ModelSettings class, ChatRequest extensions, and backend handling.

## Requirements

### Requirement: ModelSettings shared model

The Shared project SHALL define a `ModelSettings` class with nullable properties: `Temperature` (double?), `TopP` (double?), `MaxTokens` (int?). Null values indicate "use server default".

#### Scenario: All fields null

- **WHEN** a ModelSettings instance has all null fields
- **THEN** the backend uses Semantic Kernel default values for all parameters

#### Scenario: Partial override

- **WHEN** a ModelSettings instance has Temperature set but TopP and MaxTokens null
- **THEN** only Temperature is overridden; other parameters use defaults

### Requirement: System prompt in chat request

The `ChatRequest` SHALL accept an optional `SystemPrompt` (string?) property. When present and non-empty, the backend SHALL insert it as the first system message in the ChatHistory before user/assistant messages.

#### Scenario: System prompt provided

- **WHEN** a ChatRequest includes a non-empty SystemPrompt
- **THEN** the ChatHistory starts with a system message containing that text, followed by the conversation messages

#### Scenario: System prompt absent

- **WHEN** a ChatRequest has a null or empty SystemPrompt
- **THEN** the ChatHistory contains only user and assistant messages (no system message)

### Requirement: Model settings in chat request

The `ChatRequest` SHALL accept an optional `Settings` (ModelSettings?) property. When present, the backend SHALL apply non-null values to `OpenAIPromptExecutionSettings` before calling the Semantic Kernel.

#### Scenario: Temperature override

- **WHEN** a ChatRequest includes Settings with Temperature = 0.5
- **THEN** the OpenAIPromptExecutionSettings.Temperature is set to 0.5

#### Scenario: No settings provided

- **WHEN** a ChatRequest has null Settings
- **THEN** the backend uses default OpenAIPromptExecutionSettings (only FunctionChoiceBehavior.Auto is set)