Media.net adapter: support regional (geo-routed) endpoints

[anishb-ms]

Background / Motivation
Media.net currently routes all server-to-server bid traffic through a single global endpoint (https://prebid-adapter.media.net/rtb/pb/prebids2s). To reduce latency and route requests to the nearest datacenter, Media.net is introducing regional endpoints:

Geo code: USE
Region: US East
Endpoint: https://prebid-adapter-useast.media.net/rtb/pb/prebids2s

Geo code: USW
Region: US West
Endpoint: https://prebid-adapter-uswest.media.net/rtb/pb/prebids2s

Geo code: APAC
Region: Asia
Endpoint: https://prebid-adapter-asia.media.net/rtb/pb/prebids2s

Geo code: EUC
Region: Europe Central
Endpoint: https://prebid-adapter-eu.media.net/rtb/pb/prebids2s

Because Prebid Server deployments are operated by many different hosts (not Media.net), the region cannot be selected from the server’s deployment/config. It has to travel in the bid request, so the routing decision is request-driven.

Proposed implementation

1. New optional imp param region in medianet.json, constrained to an enum: ["USE", "USW", "APAC", "EUC"]. It is not required, preserving backward compatibility.

2. ExtImpMedianet (imp_medianet.go) gains a Region string json:"region,omitempty"`` field.

3. medianet.yaml endpoint becomes a template using the standard {{.Host}} macro:
   endpoint: "https://{{.Host}}/rtb/pb/prebids2s"

4. Adapter (medianet.go):

• Builder now parses the endpoint as a text/template
• The region is read from the first impression’s bidder params.

{
    "id": "test-region-use",
    "test": 1,
    "ext": { "prebid": { "debug": true } },
    "imp": [{
      "id": "1",
      "banner": { "format": [{ "w": 300, "h": 250 }] },
      "ext": {
        "prebid": {
          "bidder": {
            "medianet": { "cid": "xxxxxxx", "crid": "xxxxxxx", "region": "USE" }
          }
        }
      }
    }]
  }

• getRegionHost(region) maps the geo code to the regional host and is resolved via macros.ResolveMacros.
• The existing src query param (derived from extra_info) is preserved.
• Matching is case-insensitive; an empty or unrecognized region falls back to the current default host prebid-adapter.media.net, so existing integrations are unaffected.

Backward compatibility

• region is optional. Requests without it resolve to https://prebid-adapter.media.net/rtb/pb/prebids2s — identical to today’s behavior.
• No change to request/response bodies; region simply passes through inside imp.ext like the existing cid/crid params.

Testing

• Updated the JSON test harness endpoint to the templated host and refreshed expected URIs.
• Added an exemplary JSON exercising regional routing (region: "USE" → prebid-adapter-useast.media.net).
• Added a unit test (TestGetRegionHost) covering all four codes, case-insensitivity, and the default fallback.
• Updated the malformed-template test to expect a build error (Builder now parses a template).
• go build anish-ms. and go test ./adapters/medianet/... ./openrtb_ext/... prebid-server. all pass.