feat: Introduce a pluggable LLM provider system with token extraction, pricing, and updated setup configuration.

Providers/IPricingProvider.cs

@@ -0,0 +1,18 @@
+using AnchorCli.OpenRouter;
+namespace AnchorCli.Providers;
+
+/// <summary>
+/// Interface for fetching model pricing information.
+/// </summary>
+internal interface IPricingProvider
+{
+    /// <summary>
+    /// Fetches pricing info for a specific model.
+    /// </summary>
+    Task<ModelInfo?> GetModelInfoAsync(string modelId, CancellationToken ct = default);
+
+    /// <summary>
+    /// Fetches all available models with pricing.
+    /// </summary>
+    Task<Dictionary<string, ModelInfo>> GetAllModelsAsync(CancellationToken ct = default);
+}