THREESCALE-10151 fix proxy policy doesn't send headers set by APIcast to the API Backend

[tkan145]

What

Fix https://redhat.atlassian.net/browse/THREESCALE-10151

Verification steps:

• Checkout this branch
• Build new runtime-image

make runtime-image IMAGE_NAME=apicast-test

• Get inside dev-envinroment

cd dev-environments/https-proxy-upstream-tlsv1.3/

• Update apicast-config.json as follow

diff --git a/dev-environments/https-proxy-upstream-tlsv1.3/apicast-config.json b/dev-environments/https-proxy-upstream-tlsv1.3/apicast-config.json
index 5227c5aa..bf662f8a 100644
--- a/dev-environments/https-proxy-upstream-tlsv1.3/apicast-config.json
+++ b/dev-environments/https-proxy-upstream-tlsv1.3/apicast-config.json
@@ -4,6 +4,7 @@
       "id": "1",
       "backend_version": "1",
       "proxy": {
+        "secret_token": "token",
         "hosts": ["get.example.com"],
         "api_backend": "https://example.com/get",
         "backend": {

• Start the gateway

make gateway IMAGE_NAME=apicast-test

• Send a request

curl --resolve get.example.com:8080:127.0.0.1 -v "http://get.example.com:8080/?user_key=123"

• You should see the secret token in the response

* Request completely sent off
< HTTP/1.1 200 OK
< Content-Type: application/json
< Transfer-Encoding: chunked
< Connection: keep-alive
< Date: Mon, 08 Jun 2026 03:19:35 GMT
< Server: WEBrick/1.6.1 (Ruby/2.7.4/2021-07-07)
<
{
  "method": "GET",
  "path": "/get",
  "query_string": "user_key=123",
  "body": "",
  "headers": {
    "X-3scale-Proxy-Secret-Token": "token",
    "User-Agent": "curl/8.15.0",
    "X-3scale-Debug": "",
    "Accept": "*/*",
    "Host": "example.com",
    "X-Real-Ip": "172.20.0.1",
    "Version": "HTTP/1.1"
  },
  "uuid": "c199b276-0856-4929-a0f4-3766d8c97b3b"