refactor(channels): dedupe transport and gateway test scaffolds

src/gateway/test-openai-responses-model.ts

@@ -0,0 +1,21 @@
+export function buildOpenAiResponsesTestModel(id = "gpt-5.2") {
+  return {
+    id,
+    name: id,
+    api: "openai-responses",
+    reasoning: false,
+    input: ["text"],
+    cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+    contextWindow: 128_000,
+    maxTokens: 4096,
+  } as const;
+}
+
+export function buildOpenAiResponsesProviderConfig(baseUrl: string, modelId = "gpt-5.2") {
+  return {
+    baseUrl,
+    apiKey: "test",
+    api: "openai-responses",
+    models: [buildOpenAiResponsesTestModel(modelId)],
+  } as const;
+}