feat(compaction): support SubAPI native remote compaction

- support sub-api/openai-responses through the public Responses endpoint
- preserve explicit Authorization and support header-only authentication
- retain Codex JWT requirements and native compaction failure fallback
- restore SubAPI checkpoints and preserve portable model-switching summaries
- add provider, authentication and checkpoint lifecycle regression tests
- document compatibility requirements and the Sub2API protocol reference

pi-compaction/src/native-provider.ts

@@ -0,0 +1,66 @@
+import {
+  buildCodexHeaders,
+  isJsonObject,
+  isOpenAICodexModel,
+  mergeFeatureHeader,
+  resolveCodexResponsesUrl,
+  type Model,
+} from "./native-compaction.js";
+
+export function isNativeCompactionModel(model: unknown): model is Model<any> {
+  return isOpenAICodexModel(model) || (
+    isJsonObject(model) && model.provider === "sub-api" && model.api === "openai-responses"
+  );
+}
+
+export function buildNativeCompactionTransport(params: {
+  model: Pick<Model<any>, "provider" | "api" | "baseUrl">;
+  apiKey?: string;
+  headers?: Record<string, string>;
+  sessionId: string;
+}): { url: string; headers: Headers } {
+  if (isOpenAICodexModel(params.model)) {
+    if (!params.apiKey) throw new Error("Codex native compaction authentication requires an API key.");
+    return {
+      url: resolveCodexResponsesUrl(params.model.baseUrl),
+      headers: buildCodexHeaders({ ...params, apiKey: params.apiKey }),
+    };
+  }
+  if (!isNativeCompactionModel(params.model)) {
+    throw new Error("Native compaction is not supported for this provider and API.");
+  }
+  if (!params.model.baseUrl?.trim()) {
+    throw new Error("SubAPI native compaction requires an explicit baseUrl.");
+  }
+  let url: URL;
+  try {
+    url = new URL(params.model.baseUrl.trim());
+  } catch {
+    throw new Error("SubAPI native compaction requires a valid HTTP(S) baseUrl.");
+  }
+  if (!["https:", "http:"].includes(url.protocol) || url.username || url.password || url.hash) {
+    throw new Error("SubAPI native compaction requires an HTTP(S) baseUrl without credentials or a fragment.");
+  }
+  const path = url.pathname.replace(/\/+$/, "");
+  url.pathname = path.endsWith("/responses") ? path : `${path}/responses`;
+
+  // Sub2API accepts native compaction v2 through its public Responses endpoint.
+  // The gateway owns upstream Codex credentials; honor the client's resolved authentication.
+  const headers = new Headers(params.headers);
+  if (!headers.get("authorization")?.trim()) {
+    if (params.apiKey?.trim()) {
+      headers.set("authorization", `Bearer ${params.apiKey}`);
+    } else if (!headers.get("cf-aig-authorization")?.trim()) {
+      throw new Error("SubAPI native compaction authentication requires an API key or a non-empty Authorization or cf-aig-authorization header.");
+    }
+  }
+  headers.set("content-type", "application/json");
+  headers.set("accept", "text/event-stream");
+  headers.set("OpenAI-Beta", "responses=experimental");
+  headers.set("originator", "pi");
+  headers.set("user-agent", "pi-compaction");
+  headers.set("session-id", params.sessionId);
+  headers.set("x-client-request-id", params.sessionId);
+  headers.set("x-codex-beta-features", mergeFeatureHeader(headers.get("x-codex-beta-features")));
+  return { url: url.toString(), headers };
+}