[codex] Add model tester fixed-channel selection (#352)

* feat: add model tester forced channel

* fix: keep codex websocket defaults and route refresh checks

* fix: scope tester-only forced channel overrides

* fix: keep tester hydration and websocket hints internal

* fix: reject gemini on legacy tester chat routes

* fix: harden forced channel selection edges

src/server/proxy-core/providers/codexProviderProfile.ts

@@ -10,7 +10,7 @@ export const codexProviderProfile: ProviderProfile = {
   id: 'codex',
   prepareRequest(input: PrepareProviderRequestInput): PreparedProviderRequest {
     const isCodexOauth = asTrimmedString(input.oauthProvider).toLowerCase() === 'codex';
-    const websocketTransport = getInputHeader(input.baseHeaders, 'x-metapi-responses-websocket-transport') === '1';
+    const websocketTransport = input.responsesWebsocketTransport === true;
     const configuredUserAgent = isCodexOauth ? asTrimmedString(config.codexHeaderDefaults.userAgent) : '';
     const configuredBetaFeatures = (
       isCodexOauth && websocketTransport

src/server/proxy-core/providers/types.ts

@@ -42,6 +42,7 @@ export type PrepareProviderRequestInput = {
   claudeHeaders?: Record<string, string>;
   codexSessionCacheKey?: string | null;
   codexExplicitSessionId?: string | null;
+  responsesWebsocketTransport?: boolean;
   body: Record<string, unknown>;
   action?: ProviderAction;
 };