Fix duplicate workflow instance and execution ID fields in logs

[Copilot]

This PR fixes an issue where workflow.instance.id and workflow.execution.id fields were appearing multiple times in workflow logs, causing unnecessary duplication and potential confusion in log analysis.

Problem

The logger passed into the Executor was already enhanced with InstanceIDKey and ExecutionIDKey fields, but then these same fields were being added again in three other places:

1. Executor constructor - Added the fields again in the NewExecutor function (lines 113-116)
2. WorkflowState replay logger - Added the fields again when creating the replay logger
3. Executor event logging - Added InstanceIDKey again in executeEvent() and Close() methods

This resulted in log entries like:

{
  "msg": "Executing event",
  "workflows.instance.id": "123",
  "workflows.execution.id": "456", 
  "workflows.instance.id": "123",
  "workflows.execution.id": "456"
}

Solution

Removed all redundant field additions while keeping the primary logger enhancement that happens before the executor is created. The changes are minimal and surgical:

• Remove duplicate .With() call in executor constructor (NewExecutor)
• Remove duplicate .With() call in NewWorkflowState()
• Remove InstanceIDKey from debug logging in executeEvent()
• Remove InstanceIDKey from debug logging in Close()

Verification

• All existing tests continue to pass
• Manual testing with sample workflows confirms logs now show each field only once
• Log entries now appear correctly as:

{
  "msg": "Executing event",
  "workflows.instance.id": "123",
  "workflows.execution.id": "456"
}

Fixes #405.

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