Add GPT-5 support with Gpt_5, Gpt_5_turbo, and Gpt_5_mini models

[Copilot]

This PR adds comprehensive support for GPT-5 models to the Betalgo OpenAI library, enabling developers to use OpenAI's latest language model variants.

Changes Made

New Model Support:

• Models.Gpt_5 → "gpt-5" - Base GPT-5 model with enhanced reasoning and collaboration capabilities
• Models.Gpt_5_turbo → "gpt-5-turbo" - Optimized version with improved speed and efficiency
• Models.Gpt_5_mini → "gpt-5-mini" - Affordable and faster variant for simpler tasks

Implementation Details:

• Added enum values to Models.Model enum following existing patterns
• Added static string properties with comprehensive XML documentation
• Added proper enum-to-string mappings in EnumToString() method
• Updated README with GPT-5 usage example

Usage Example

var completionResult = await openAiService.ChatCompletion.CreateCompletion(new ChatCompletionCreateRequest
{
    Messages = new List<ChatMessage>
    {
        ChatMessage.FromSystem("You are a helpful assistant powered by GPT-5."),
        ChatMessage.FromUser("What are the key advantages of GPT-5?"),
    },
    Model = Models.Gpt_5, // Available: Gpt_5, Gpt_5_turbo, Gpt_5_mini
});
if (completionResult.Successful)
{
    Console.WriteLine(completionResult.Choices.First().Message.Content);
}

The implementation follows the existing codebase patterns and maintains full compatibility with the current API structure. All GPT-5 models are now available for use in chat completions and other OpenAI services.

Fixes #733.

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