refactor: Update BaseClient to handle non-ephemeral agents in conversation logic

- Added a check for non-ephemeral agents in BaseClient, modifying the exceptions set to include 'model' when applicable.
- Enhanced conversation handling to improve flexibility based on agent type.

api/app/clients/BaseClient.js

@@ -966,6 +966,11 @@ class BaseClient {
 
     const unsetFields = {};
     const exceptions = new Set(['spec', 'iconURL']);
+    const hasNonEphemeralAgent =
+      endpointOptions?.agent_id && endpointOptions.agent_id !== Constants.EPHEMERAL_AGENT_ID;
+    if (hasNonEphemeralAgent) {
+      exceptions.add('model');
+    }
     if (existingConvo != null) {
       this.fetchedConvo = true;
       for (const key in existingConvo) {