fix: handle HTML responses in panel management API requests

[jakub961241]

Summary

Fixes #12260 - Multi-machine management fails when adding a panel with error:

load current config failed, err: json umarshal resp data failed, err: invalid character '<' looking for beginning of value

Root Cause

When the remote panel returns a 401 Unauthorized response, it sends an HTML page (401.html) with Content-Type: text/html. The code at core/utils/req_helper/proxy_local/req_to_local.go attempted to json.Unmarshal() this HTML, causing the invalid character '<' error.

Fix

• Add explicit 401 Unauthorized handling with a clear error message
• Validate Content-Type header is JSON before attempting unmarshal
• Include response preview in error messages for easier debugging
• Read body before status check so error messages can include response content

Changed file

• core/utils/req_helper/proxy_local/req_to_local.go

fix: Fix multi-machine panel management failing with JSON unmarshal error when remote panel returns HTML response (#12260)

[f2c-ci-robot]

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