Fix duplicate prompt expressions when operating alert rule
Fix duplicate prompt expressions when operating alert rule

**Problem:**
When adding/editing alert rules in project-level alerting, there are two
same prompt expressions in Expression selection.

**Solution:**
Using String set to merge the same expression.

**Issue:**
https://github.com/rancher/rancher/issues/17999

rancherio/rancher,rancher/rancher,cjellick/rancher,rancher/rancher,rancher/rancher,cjellick/rancher,rancher/rancher,cjellick/rancher,rancherio/rancher