acp: add regression coverage and smoke-test docs

extensions/acpx/src/runtime.test.ts

@@ -127,6 +127,40 @@ describe("AcpxRuntime", () => {
     expect(promptArgs).toContain("--approve-all");
   });
 
+  it("serializes text plus image attachments into ACP prompt blocks", async () => {
+    const { runtime, logPath } = await createMockRuntimeFixture();
+
+    const handle = await runtime.ensureSession({
+      sessionKey: "agent:codex:acp:with-image",
+      agent: "codex",
+      mode: "persistent",
+    });
+
+    for await (const _event of runtime.runTurn({
+      handle,
+      text: "describe this image",
+      attachments: [{ mediaType: "image/png", data: "aW1hZ2UtYnl0ZXM=" }],
+      mode: "prompt",
+      requestId: "req-image",
+    })) {
+      // Consume stream to completion so prompt logging is finalized.
+    }
+
+    const logs = await readMockRuntimeLogEntries(logPath);
+    const prompt = logs.find(
+      (entry) =>
+        entry.kind === "prompt" &&
+        String(entry.sessionName ?? "") === "agent:codex:acp:with-image",
+    );
+    expect(prompt).toBeDefined();
+
+    const stdinBlocks = JSON.parse(String(prompt?.stdinText ?? ""));
+    expect(stdinBlocks).toEqual([
+      { type: "text", text: "describe this image" },
+      { type: "image", mimeType: "image/png", data: "aW1hZ2UtYnl0ZXM=" },
+    ]);
+  });
+
   it("preserves leading spaces across streamed text deltas", async () => {
     const runtime = sharedFixture?.runtime;
     expect(runtime).toBeDefined();