Return /mdw/v3/api with `content-type: application/json` instead of `content-type: text/plain`

[davidyuk]

curl https://mainnet.aeternity.io/mdw/v3/api -v
* Host mainnet.aeternity.io:443 was resolved.
* IPv6: (none)
* IPv4: 3.163.158.90, 3.163.158.96, 3.163.158.15, 3.163.158.114
*   Trying 3.163.158.90:443...
* Connected to mainnet.aeternity.io (3.163.158.90) port 443
* ALPN: curl offers h2,http/1.1
* (304) (OUT), TLS handshake, Client hello (1):
*  CAfile: /etc/ssl/cert.pem
*  CApath: none
* (304) (IN), TLS handshake, Server hello (2):
* (304) (IN), TLS handshake, Unknown (8):
* (304) (IN), TLS handshake, Certificate (11):
* (304) (IN), TLS handshake, CERT verify (15):
* (304) (IN), TLS handshake, Finished (20):
* (304) (OUT), TLS handshake, Finished (20):
* SSL connection using TLSv1.3 / AEAD-AES128-GCM-SHA256 / [blank] / UNDEF
* ALPN: server accepted h2
* Server certificate:
*  subject: CN=*.aeternity.io
*  start date: Jul 18 00:00:00 2025 GMT
*  expire date: Aug 15 23:59:59 2026 GMT
*  subjectAltName: host "mainnet.aeternity.io" matched cert's "*.aeternity.io"
*  issuer: C=US; O=Amazon; CN=Amazon RSA 2048 M03
*  SSL certificate verify ok.
* using HTTP/2
* [HTTP/2] [1] OPENED stream for https://mainnet.aeternity.io/mdw/v3/api
* [HTTP/2] [1] [:method: GET]
* [HTTP/2] [1] [:scheme: https]
* [HTTP/2] [1] [:authority: mainnet.aeternity.io]
* [HTTP/2] [1] [:path: /mdw/v3/api]
* [HTTP/2] [1] [user-agent: curl/8.7.1]
* [HTTP/2] [1] [accept: */*]
> GET /mdw/v3/api HTTP/2
> Host: mainnet.aeternity.io
> User-Agent: curl/8.7.1
> Accept: */*
>
* Request completely sent off
< HTTP/2 200
< content-type: text/plain; charset=utf-8
< access-control-allow-credentials: true
< access-control-allow-headers: *
< access-control-allow-origin: *
< access-control-expose-headers:
< cache-control: max-age=0, private, must-revalidate
< date: Fri, 08 Aug 2025 08:09:04 GMT
< server: Caddy
< via: 1.1 Caddy, 1.1 8e6c2cf5874f5e4093136cc3de4d856a.cloudfront.net (CloudFront)
< x-ip: 64.252.71.13
< x-proxy-client-ip: 217.150.77.56
< x-proxy-server: 172.31.37.244
< x-real-ip: 217.150.77.56
< x-request-id: GFm8R21lMGFJzRMAI-oh
< vary: Accept-Encoding
< x-cache: Miss from cloudfront
< x-amz-cf-pop: SEA900-P1
< x-amz-cf-id: QieeT2bCTwD3oWM2swBI0N5JbE1qHSMDjCp5DoNG6_Wn7j7eNmxqAQ==
<
{
  "openapi": "3.0.0",
  "info": {
    "description": "API for [Aeternity Middleware](https://github.com/aeternity/ae_mdw)",
    "title": "Aeternity Middleware",
    "version": "1.104.3"