Issue description

When editing upstream nodes in APISIX Dashboard (adding, removing, or modifying nodes), the pass_host field is automatically reset to the default value "pass", instead of preserving the previously configured value (e.g., "node").

This causes serious issues when the backend is a Kubernetes Ingress or reverse proxy that relies on the Host header for routing - requests fail with 404 Not Found because the wrong Host header is sent.

Environment:

• APISIX version: 3.2.2 LTS
• APISIX Dashboard version: 3.0.1
• etcd version: 3.5.7

Related Issues:

• Discovery Args are droped when edit upstream #3270 (Discovery Args are dropped when edit upstream) - Similar pattern
• Plugin with empty json config is dropped when add or edit Route #3269 (Plugin config is dropped when edit Route) - Similar pattern

Expected behavior

When editing upstream nodes, the pass_host field should retain its previously configured value (e.g., "node"), not be reset to the default value "pass".

How to Reproduce

1. Create an upstream via Admin API or Dashboard with:

   • Node: my-service.my-namespace.svc:80 (domain-based)
   • pass_host: "node"

2. Verify the upstream works correctly (backend receives proper Host header)

3. Edit the upstream in Dashboard - add or modify a node

4. Save the upstream

5. Check the configuration via Admin API:
   curl -s http://127.0.0.1:9180/apisix/admin/upstreams/{id} -H 'X-API-KEY: xxx'

6. Observe that pass_host has been reset to "pass" instead of remaining "node"

Screenshots

No response

Environment

• apisix version (cmd: apisix version):
• OS (cmd: uname -a):
• OpenResty / Nginx version (cmd: nginx -V or openresty -V):
• etcd version, if have (cmd: run etcd --version):
• apisix-dashboard version, if have:
• Browser version, if have:

Additional context

No response