feat: pass accept header to oauth2-proxy

[merll]

📌 Summary

This changes the configuration of the Istio backend interface to OAuth2-Proxy, which is expected to return JSON and not HTML on errors.

🔍 Reviewer Notes

🧹 Checklist

• Code is readable, maintainable, and robust.
• Unit tests added/updated

[svcAPLBot]

Comparison of Helm chart templating output:

@@ data.original-values @@
! ± value change in multiline text (one insert, one deletion)
  {
    "autoscaleMax": 5,
    "autoscaleMin": 1,
    "env": {
  
  [77 lines unchanged)]
  
              "x-auth-request-user"
            ],
            "includeRequestHeadersInCheck": [
              "authorization",
-             "cookie"
+             "cookie",
+             "accept"
            ],
            "port": 80,
            "service": "oauth2-proxy.istio-system.svc.cluster.local"
          },
  
  [27 lines unchanged)]
  
      }
    },
    "revision": "1-26-0"
  }

@@ data.merged-values @@
! ± value change in multiline text (one insert, one deletion)
  {
    "affinity": {},
    "autoscaleBehavior": {},
    "autoscaleEnabled": true,
  
  [206 lines unchanged)]
  
              "x-auth-request-user"
            ],
            "includeRequestHeadersInCheck": [
              "authorization",
-             "cookie"
+             "cookie",
+             "accept"
            ],
            "port": 80,
            "service": "oauth2-proxy.istio-system.svc.cluster.local"
          },
  
  [78 lines unchanged)]
  
    "variant": "",
    "volumeMounts": [],
    "volumes": []
  }


@@ data.mesh @@
! ± value change in multiline text (one insert, no deletions)
  accessLogFile: /dev/stdout
  accessLogFormat: |
    [%START_TIME%] "%REQ(:METHOD)% %REQ(X-ENVOY-ORIGINAL-PATH?:PATH)% %PROTOCOL%" %RESPONSE_CODE% %RESPONSE_FLAGS% %RESPONSE_CODE_DETAILS% %CONNECTION_TERMINATION_DETAILS% "%UPSTREAM_TRANSPORT_FAILURE_REASON%" %BYTES_RECEIVED% %BYTES_SENT% %DURATION% %RESP(X-ENVOY-UPSTREAM-SERVICE-TIME)% "%REQ(X-FORWARDED-FOR)%" "%REQ(USER-AGENT)%" "%REQ(X-REQUEST-ID)%" "%REQ(:AUTHORITY)%" "%UPSTREAM_HOST%" %UPSTREAM_CLUSTER% %UPSTREAM_LOCAL_ADDRESS% %DOWNSTREAM_LOCAL_ADDRESS% %DOWNSTREAM_REMOTE_ADDRESS% %REQUESTED_SERVER_NAME% %ROUTE_NAME% traceID=%REQ(TRACEPARENT)%
  defaultConfig:
  
  [25 lines unchanged)]
  
      - x-auth-request-user
      includeRequestHeadersInCheck:
      - authorization
      - cookie
+     - accept
      port: 80
      service: oauth2-proxy.istio-system.svc.cluster.local
    name: oauth2-proxy-ext-authz
  - envoyExtAuthzGrpc:
  
  [seven lines unchanged)]
  
    interval: 5s
    probes: 3
    time: 10s
  trustDomain: cluster.local