Add parameter validation to CreateSessionAsync methods

Co-authored-by: westey-m <164392973+westey-m@users.noreply.github.com>

Author: Copilot

dotnet/src/Microsoft.Agents.AI.A2A/A2AAgent.cs

@@ -63,7 +63,10 @@ protected sealed override ValueTask<AgentSession> CreateSessionCoreAsync(Cancell
     /// <param name="contextId">The context id to continue.</param>
     /// <returns>A value task representing the asynchronous operation. The task result contains a new <see cref="AgentSession"/> instance.</returns>
     public ValueTask<AgentSession> CreateSessionAsync(string contextId)
-        => new(new A2AAgentSession() { ContextId = contextId });
+    {
+        _ = Throw.IfNullOrWhitespace(contextId);
+        return new(new A2AAgentSession() { ContextId = contextId });
+    }
 
     /// <summary>
     /// Get a new <see cref="AgentSession"/> instance using an existing context id and task id, to resume that conversation from a specific task.
@@ -72,7 +75,11 @@ public ValueTask<AgentSession> CreateSessionAsync(string contextId)
     /// <param name="taskId">The task id to resume from.</param>
     /// <returns>A value task representing the asynchronous operation. The task result contains a new <see cref="AgentSession"/> instance.</returns>
     public ValueTask<AgentSession> CreateSessionAsync(string contextId, string taskId)
-        => new(new A2AAgentSession() { ContextId = contextId, TaskId = taskId });
+    {
+        _ = Throw.IfNullOrWhitespace(contextId);
+        _ = Throw.IfNullOrWhitespace(taskId);
+        return new(new A2AAgentSession() { ContextId = contextId, TaskId = taskId });
+    }
 
     /// <inheritdoc/>
     protected override ValueTask<JsonElement> SerializeSessionCoreAsync(AgentSession session, JsonSerializerOptions? jsonSerializerOptions = null, CancellationToken cancellationToken = default)

dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/A2AAgentTests.cs

@@ -1199,7 +1199,7 @@ public async Task CreateSessionAsync_WithContextIdAndTaskId_CreatesSessionWithBo
     public async Task CreateSessionAsync_WithInvalidContextId_ThrowsArgumentExceptionAsync(string? contextId)
     {
         // Act & Assert
-        await Assert.ThrowsAsync<ArgumentException>(async () =>
+        await Assert.ThrowsAnyAsync<ArgumentException>(async () =>
             await this._agent.CreateSessionAsync(contextId!));
     }
 
@@ -1218,7 +1218,7 @@ public async Task CreateSessionAsync_WithInvalidContextIdAndValidTaskId_ThrowsAr
         const string TaskId = "valid-task-id";
 
         // Act & Assert
-        await Assert.ThrowsAsync<ArgumentException>(async () =>
+        await Assert.ThrowsAnyAsync<ArgumentException>(async () =>
             await this._agent.CreateSessionAsync(contextId!, TaskId));
     }
 
@@ -1237,7 +1237,7 @@ public async Task CreateSessionAsync_WithValidContextIdAndInvalidTaskId_ThrowsAr
         const string ContextId = "valid-context-id";
 
         // Act & Assert
-        await Assert.ThrowsAsync<ArgumentException>(async () =>
+        await Assert.ThrowsAnyAsync<ArgumentException>(async () =>
             await this._agent.CreateSessionAsync(ContextId, taskId!));
     }
     #endregion