Introduce "Safe" SystemDescription (microsoft#589)

### Motivation and Context
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Stored system prompt references `TimeSkill `plugin that has been renamed
to `TimePlugin`.

System prompts for each chat have been stored in cosmosdb.


### Description
<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

Dynamically intercept system-prompt and replace plugin reference.

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [Contribution
Guidelines](https://github.com/microsoft/chat-copilot/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/chat-copilot/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

webapi/Controllers/ChatArchiveController.cs

@@ -99,7 +99,7 @@ private async Task<ChatArchive> CreateChatArchiveAsync(Guid chatId, Cancellation
         chatArchive.ChatTitle = chat.Title;
 
         // get the system description
-        chatArchive.SystemDescription = chat.SystemDescription;
+        chatArchive.SystemDescription = chat.SafeSystemDescription;
 
         // get the chat history
         chatArchive.ChatHistory = await this.GetAllChatMessagesAsync(chatIdString);

webapi/Controllers/ChatHistoryController.cs

@@ -218,7 +218,7 @@ public async Task<IActionResult> EditChatSessionAsync(
         if (await this._sessionRepository.TryFindByIdAsync(chatId.ToString(), callback: v => chat = v))
         {
             chat!.Title = chatParameters.Title ?? chat!.Title;
-            chat!.SystemDescription = chatParameters.SystemDescription ?? chat!.SystemDescription;
+            chat!.SystemDescription = chatParameters.SystemDescription ?? chat!.SafeSystemDescription;
             chat!.MemoryBalance = chatParameters.MemoryBalance ?? chat!.MemoryBalance;
             await this._sessionRepository.UpsertAsync(chat);
             await messageRelayHubContext.Clients.Group(chatId.ToString()).SendAsync(ChatEditedClientCall, chat);

webapi/Models/Storage/ChatSession.cs

@@ -34,6 +34,11 @@ public class ChatSession : IStorageEntity
     /// </summary>
     public string SystemDescription { get; set; }
 
+    /// <summary>
+    /// Fixed system description with "TimeSkill" replaced by "TimePlugin"
+    /// </summary>
+    public string SafeSystemDescription => this.SystemDescription.Replace("TimeSkill", "TimePlugin", StringComparison.OrdinalIgnoreCase);
+
     /// <summary>
     /// The balance between long term memory and working term memory.
     /// The higher this value, the more the system will rely on long term memory by lowering

webapi/Plugins/Chat/ChatPlugin.cs

@@ -1024,6 +1024,6 @@ private async Task SetSystemDescriptionAsync(string chatId, CancellationToken ca
             throw new ArgumentException("Chat session does not exist.");
         }
 
-        this._promptOptions.SystemDescription = chatSession!.SystemDescription;
+        this._promptOptions.SystemDescription = chatSession!.SafeSystemDescription;
     }
 }