feat: Introduce an OpenAI-compatible client to replace the Groq-specific client and enable multiple LLM providers.

Api/Models/OpenAiModels.cs

@@ -0,0 +1,56 @@
+using System.Text.Json.Serialization;
+
+namespace Toak.Api.Models;
+
+public class OpenAiRequestMessage
+{
+    [JsonPropertyName("role")]
+    public string Role { get; set; } = string.Empty;
+    [JsonPropertyName("content")]
+    public string Content { get; set; } = string.Empty;
+}
+
+public class OpenAiRequest
+{
+    [JsonPropertyName("model")]
+    public string Model { get; set; } = "llama-3.1-8b-instant";
+    [JsonPropertyName("messages")]
+    public OpenAiRequestMessage[] Messages { get; set; } = Array.Empty<OpenAiRequestMessage>();
+    [JsonPropertyName("temperature")]
+    public double Temperature { get; set; } = 0.0;
+    [JsonPropertyName("stream")]
+    public bool? Stream { get; set; }
+    [JsonPropertyName("reasoning_effort")]
+    public string? ReasoningEffort { get; set; }
+}
+
+public class OpenAiResponse
+{
+    [JsonPropertyName("choices")]
+    public OpenAiChoice[] Choices { get; set; } = Array.Empty<OpenAiChoice>();
+}
+
+public class OpenAiChoice
+{
+    [JsonPropertyName("message")]
+    public OpenAiRequestMessage Message { get; set; } = new();
+}
+
+public class OpenAiStreamResponse
+{
+    [JsonPropertyName("choices")]
+    public OpenAiStreamChoice[] Choices { get; set; } = Array.Empty<OpenAiStreamChoice>();
+}
+
+public class OpenAiStreamChoice
+{
+    [JsonPropertyName("delta")]
+    public OpenAiStreamDelta Delta { get; set; } = new();
+}
+
+public class OpenAiStreamDelta
+{
+    [JsonPropertyName("content")]
+    public string? Content { get; set; }
+}
+