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/MistralChatCompletionRequest.cs

@@ -0,0 +1,31 @@
+using System.Text.Json.Serialization;
+
+namespace Luna.Providers.Mistral.DTOs;
+
+public class MistralChatCompletionRequest
+{
+    [JsonPropertyName("model")]
+    public required string Model { get; set; }
+
+    [JsonPropertyName("messages")]
+    public required List<MistralMessage> Messages { get; set; }
+
+    [JsonPropertyName("temperature")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public float? Temperature { get; set; }
+
+    [JsonPropertyName("max_tokens")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public int? MaxTokens { get; set; }
+
+    [JsonPropertyName("top_p")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public float? TopP { get; set; }
+
+    [JsonPropertyName("stream")]
+    public bool Stream { get; set; }
+
+    [JsonPropertyName("response_format")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public ResponseFormat? ResponseFormat { get; set; }
+}