fix(security): non-thread-safe sequence number generation may cau (#5320)

`SequenceNumber.Increment()` uses `this._sequenceNumber++` without synchronization. In concurrent streaming scenarios, this can produce race conditions and inconsistent sequencing, which may break event ordering guarantees and potentially allow response-mixing or state confusion.

Affected files: SequenceNumber.cs

Signed-off-by: tuanaiseo <221258316+tuanaiseo@users.noreply.github.com>
Co-authored-by: Jacob Alber <jaalber@microsoft.com>

dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/Responses/Streaming/SequenceNumber.cs

@@ -13,5 +13,5 @@ internal sealed class SequenceNumber
     /// Gets the next sequence number.
     /// </summary>
     /// <returns>The next sequence number.</returns>
-    public int Increment() => this._sequenceNumber++;
+    public int Increment() => System.Threading.Interlocked.Increment(ref this._sequenceNumber) - 1;
 }