admin: fix warnings and status code in /load API

[amirdaraby]

Description

This PR fixes #7246
where the Admin API /load endpoint returned 200 OK instead of 400 Bad Request when an invalid Caddyfile also produced adapter warnings.

The problem happened because the warnings were written to the response first, which made http.ResponseWriter default to 200 OK. As a result, the response contained two separate JSON objects (warnings + error) stuck together, which was invalid JSON.

Changes

• add "warnings" as valid JSON structure alongside to "errors" in JSON response.
• Make sure the correct status code (400 Bad Request) is returned for invalid configs.

Behavior Before

curl -v --unix-socket /run/uncloud/caddy/admin.sock \
  -H "Content-Type: text/caddyfile" \
  --data-binary @Caddyfile \
  http://localhost/load
*   Trying /run/uncloud/caddy/admin.sock:0...
* Connected to localhost (/run/uncloud/caddy/admin.sock) port 0
* using HTTP/1.x
> POST /load HTTP/1.1
> Host: localhost
> User-Agent: curl/8.14.1
> Accept: */*
> Content-Type: text/caddyfile
> Content-Length: 81
> 
* upload completely sent off: 81 bytes
< HTTP/1.1 200 OK
< Date: Fri, 19 Sep 2025 15:21:18 GMT
< Content-Length: 336
< Content-Type: text/plain; charset=utf-8
< Connection: close
< 
[{"file":"Caddyfile","line":2,"message":"Caddyfile input is not formatted; run 'caddy fmt --overwrite' to fix inconsistencies"}]{"error":"loading config: loading new config: loading http app module: provision http: getting tls app: loading tls app module: provision tls: loading certificates: open cert.pem: no such file or directory"}
* shutting down connection #0

Behavior After

curl -v --unix-socket /run/uncloud/caddy/admin.sock \
  -H "Content-Type: text/caddyfile" \
  --data-binary @Caddyfile \
  http://localhost/load
*   Trying /run/uncloud/caddy/admin.sock:0...
* Connected to localhost (/run/uncloud/caddy/admin.sock) port 0
* using HTTP/1.x
> POST /load HTTP/1.1
> Host: localhost
> User-Agent: curl/8.14.1
> Accept: */*
> Content-Type: text/caddyfile
> Content-Length: 81
> 
* upload completely sent off: 81 bytes
< HTTP/1.1 400 Bad Request
< Date: Fri, 19 Sep 2025 15:26:41 GMT
< Content-Length: 348
< Content-Type: text/plain; charset=utf-8
< Connection: close
< 
{"error":"loading config: loading new config: loading http app module: provision http: getting tls app: loading tls app module: provision tls: loading certificates: open cert.pem: no such file or directory","warnings":[{"file":"Caddyfile","line":2,"message":"Caddyfile input is not formatted; run 'caddy fmt --overwrite' to fix inconsistencies"}]}
* shutting down connection #0

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