Add provider abstraction layer with Mistral and Ollama implementations

Define IProvider interface and implement concrete providers for Mistral
(with custom DTOs for chat completion API) and Ollama (via OllamaSharp).
Includes DI registration extensions using Microsoft.Extensions.AI.

Luna.Providers/Mistral/DTOs/MistralChoice.cs

@@ -0,0 +1,15 @@
+using System.Text.Json.Serialization;
+
+namespace Luna.Providers.Mistral.DTOs;
+
+public class MistralChoice
+{
+    [JsonPropertyName("index")]
+    public int Index { get; set; }
+
+    [JsonPropertyName("message")]
+    public MistralMessage? Message { get; set; }
+
+    [JsonPropertyName("finish_reason")]
+    public string? FinishReason { get; set; }
+}