Fix keyset pagination with monotonic UUIDv7-like task IDs (#1215)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/Common/Polyfills/System/IdHelpers.cs

@@ -0,0 +1,40 @@
+using System.Threading;
+
+namespace System;
+
+/// <summary>
+/// Provides helper methods for monotonic ID generation.
+/// </summary>
+internal static class IdHelpers
+{
+    private static long s_counter;
+
+    /// <summary>
+    /// Creates a strictly monotonically increasing identifier string using 64-bit timestamp ticks
+    /// and a 64-bit counter, formatted as a 32-character hexadecimal string (GUID-like).
+    /// </summary>
+    /// <param name="timestamp">The timestamp to embed in the identifier.</param>
+    /// <returns>A new strictly monotonically increasing identifier string.</returns>
+    /// <remarks>
+    /// <para>
+    /// This method creates a 128-bit identifier composed of two 64-bit values:
+    /// - High 64 bits: <see cref="DateTimeOffset.Ticks"/> from the timestamp
+    /// - Low 64 bits: A globally monotonically increasing counter
+    /// </para>
+    /// <para>
+    /// The resulting string is strictly monotonically increasing when compared lexicographically,
+    /// which is required for keyset pagination to work correctly. Unlike <c>Guid.CreateVersion7</c>,
+    /// which uses random bits for intra-millisecond uniqueness, this implementation guarantees
+    /// strict ordering for all identifiers regardless of when they were created.
+    /// </para>
+    /// </remarks>
+    public static string CreateMonotonicId(DateTimeOffset timestamp)
+    {
+        long ticks = timestamp.UtcTicks;
+        long counter = Interlocked.Increment(ref s_counter);
+
+        // Format as 32-character hex string (16 bytes = 128 bits)
+        // High 64 bits: timestamp ticks, Low 64 bits: counter
+        return $"{ticks:x16}{counter:x16}";
+    }
+}