agent preview start --authoring-bundle crashes for AgentforceEmployeeAgent without default_agent_user (reading 'replace') #3676

Summary

sf agent preview start --authoring-bundle <name> crashes for an Agent Script agent of type AgentforceEmployeeAgent that has no default_agent_user in its config block (employee agents run as the logged-in user, so this field is not set).

Steps to reproduce

1. Agent Script authoring bundle with:

   config:
       developer_name: "My_Employee_Agent"
       agent_type: "AgentforceEmployeeAgent"
   

   (no default_agent_user)
2. sf agent validate authoring-bundle --api-name My_Employee_Agent → success
3. sf agent preview start --authoring-bundle My_Employee_Agent --use-live-actions --target-org <sandbox> --json

Expected result

A preview session starts (as it does for sf agent preview start --api-name My_Employee_Agent on the published, activated version — that works fine).

Actual result

PreviewStartFailed: Failed to start preview session: Cannot read properties of undefined (reading 'replace')
  cause: TypeError: Cannot read properties of undefined (reading 'replace')
      at ScriptAgent.startPreview (@salesforce/agents/lib/agents/scriptAgent.js:391:93)

Cause

@salesforce/agents/lib/agents/scriptAgent.js line 391:

const escapedDefaultAgentUser = this.agentJson.globalConfiguration.defaultAgentUser.replace(/'/g, "''");

defaultAgentUser is undefined for employee agents without default_agent_user. A guard such as (defaultAgentUser ?? '') — or skipping the user lookup when it is undefined, since bypassUser is forced to false for AgentforceEmployeeAgent a few lines later anyway — would fix it.

System information

• @salesforce/cli/2.156.8 darwin-arm64 node-v24.22.1
• @salesforce/plugin-agent 2.2.3, @salesforce/agents 2.3.1
• Also reproduced on @salesforce/cli/2.154.4
• Target org: sandbox, Agentforce employee agent