Use monotonic UUID v7 for task IDs with polyfill for older targets

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Server/InMemoryMcpTaskStore.cs

@@ -448,7 +448,63 @@ public void Dispose()
         _cleanupTimer?.Dispose();
     }
 
-    private static string GenerateTaskId() => Guid.NewGuid().ToString("N");
+#if NET9_0_OR_GREATER
+    private static string GenerateTaskId() => Guid.CreateVersion7().ToString("N");
+#else
+    private static string GenerateTaskId() => CreateVersion7Guid().ToString("N");
+
+    /// <summary>
+    /// Polyfill for Guid.CreateVersion7() on targets earlier than .NET 9.
+    /// Generates a UUID v7 with embedded timestamp for monotonic ordering.
+    /// </summary>
+    private static Guid CreateVersion7Guid()
+    {
+        // UUID v7 format (RFC 9562):
+        // - 48 bits: Unix timestamp in milliseconds (big-endian)
+        // - 4 bits: version (0111 = 7)
+        // - 12 bits: random
+        // - 2 bits: variant (10)
+        // - 62 bits: random
+
+        var timestamp = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
+        byte[] bytes = new byte[16];
+
+        // Fill with random data first
+#if NETSTANDARD2_0
+        using (var rng = System.Security.Cryptography.RandomNumberGenerator.Create())
+        {
+            rng.GetBytes(bytes);
+        }
+#else
+        System.Security.Cryptography.RandomNumberGenerator.Fill(bytes);
+#endif
+
+        // Set timestamp (48 bits, big-endian) in first 6 bytes
+        bytes[0] = (byte)(timestamp >> 40);
+        bytes[1] = (byte)(timestamp >> 32);
+        bytes[2] = (byte)(timestamp >> 24);
+        bytes[3] = (byte)(timestamp >> 16);
+        bytes[4] = (byte)(timestamp >> 8);
+        bytes[5] = (byte)timestamp;
+
+        // Set version 7 (0111) in high nibble of byte 6
+        bytes[6] = (byte)((bytes[6] & 0x0F) | 0x70);
+
+        // Set variant (10) in high 2 bits of byte 8
+        bytes[8] = (byte)((bytes[8] & 0x3F) | 0x80);
+
+        // Convert from big-endian byte array to Guid
+        // Guid constructor expects bytes in a specific order for the first 8 bytes
+        // (little-endian for the first three components on Windows)
+        // We need to swap bytes to match the Guid's internal layout
+        return new Guid(
+            (int)(bytes[0] << 24 | bytes[1] << 16 | bytes[2] << 8 | bytes[3]),  // a (big-endian to int)
+            (short)(bytes[4] << 8 | bytes[5]),  // b (big-endian to short)
+            (short)(bytes[6] << 8 | bytes[7]),  // c (big-endian to short)
+            bytes[8], bytes[9], bytes[10], bytes[11],
+            bytes[12], bytes[13], bytes[14], bytes[15]);
+    }
+#endif
 
     private static bool IsTerminalStatus(McpTaskStatus status) =>
         status is McpTaskStatus.Completed or McpTaskStatus.Failed or McpTaskStatus.Cancelled;