In response to this:

…nt served metadata.

What type of PR is this?

/area conformance-machinery

What this PR does / why we need it:

This PR introduces the necessary components within the Endpoint Picker (EPP) to support a new conformance test that
verifies gateway compliance with the x-gateway-destination-endpoint-served metadata requirement.

Exposes the "x-conformance-test-served-endpoint"

following is an example

❯ curl -i http://${IP}/v1/chat/completions \
 -H "Content-Type: application/json" \
 -d '{
   "model": "'"${MODEL_NAME}"'",
   "messages": [
     {
       "role": "user",
       "content": "hi"
     }
   ]
 }'
HTTP/1.1 200 OK
x-went-into-resp-headers: true
date: Thu, 13 Nov 2025 07:24:41 GMT
server: uvicorn
x-conformance-test-served-endpoint: 10.116.3.5:8000
content-type: application/json
via: 1.1 google
transfer-encoding: chunked

{"choices":[{"finish_reason":"stop","index":0,"logprobs":null,"message":{"annotations":null,"audio":null,"content":"\u003cthink\u003e\nOkay, the user said \"hi\". I need to respond appropriately. Let me think. A simple \"Hello!\" is a good start. But maybe they want a more personal message. I should check if they have any specific request in mind. If not, a general greeting is best. I should make sure the response is friendly and open to further conversation. Let me put that into a natural reply.\n\u003c/think\u003e\n\nHello! How can I assist you today? 😊","function_call":null,"reasoning_content":null,"refusal":null,"role":"assistant","tool_calls":[]},"stop_reason":null,"token_ids":null}],"created":1763018681,"id":"chatcmpl-0535f422-84f2-479c-93d5-ca6129e657bb","kv_transfer_params":null,"model":"Qwen/Qwen3-0.6B","object":"chat.completion","prompt_logprobs":null,"prompt_token_ids":null,"service_tier":null,"system_fingerprint":null,"usage":{"completion_tokens":97,"prompt_tokens":9,"prompt_tokens_details":null,"total_tokens":106}}% 

Which issue(s) this PR fixes:

Fixes partially #1670

Does this PR introduce a user-facing change?:

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• OWNERS [nirrozenbaum]

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