feat: expose RabbitMQ delivery context

Add backward-compatible context-aware handling and permanent validation hooks.

Ref: IT-1033

Hrynco.RabbitMq/RabbitMqMessageContext.cs

@@ -0,0 +1,23 @@
+namespace Hrynco.RabbitMq;
+
+using System.Collections.Generic;
+
+/// <summary>
+/// Transport metadata supplied by RabbitMQ for a delivered message.
+/// Application-specific consumers can use this context for validation,
+/// correlation, and diagnostics without taking ownership of acknowledgements.
+/// </summary>
+public sealed record RabbitMqMessageContext
+{
+    public required string QueueName { get; init; }
+    public required string Exchange { get; init; }
+    public required string RoutingKey { get; init; }
+    public string? MessageId { get; init; }
+    public string? MessageType { get; init; }
+    public string? CorrelationId { get; init; }
+    public string? ReplyTo { get; init; }
+    public string? ContentType { get; init; }
+    public bool Redelivered { get; init; }
+    public IReadOnlyDictionary<string, object?> Headers { get; init; }
+        = new Dictionary<string, object?>();
+}