Clarify exception handling in ConversationId property (#1457)

Update XML documentation to clarify exception behavior.

See `ChatClientAgentThreadTests.SetConversationIdThrowsWhenMessageStoreIsSet` which already verifies this is the actual behavior.

Co-authored-by: Chris <[email protected]>

Author: kzu

dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgentThread.cs

@@ -75,8 +75,8 @@ internal ChatClientAgentThread(
     /// <remarks>
     /// <para>
     /// Note that either <see cref="ConversationId"/> or <see cref="MessageStore "/> may be set, but not both.
-    /// If <see cref="MessageStore "/> is not null, and <see cref="ConversationId"/> is set, <see cref="MessageStore "/>
-    /// will be reverted to null, and vice versa.
+    /// If <see cref="MessageStore "/> is not null, setting <see cref="ConversationId"/> will throw an
+    /// <see cref="InvalidOperationException "/> exception.
     /// </para>
     /// <para>
     /// This property may be null in the following cases:
@@ -91,6 +91,7 @@ internal ChatClientAgentThread(
     /// to fork the thread with each iteration.
     /// </para>
     /// </remarks>
+    /// <exception cref="InvalidOperationException">Attempted to set a conversation ID but a <see cref="MessageStore"/> is already set.</exception>
     public string? ConversationId
     {
         get => this._conversationId;