fix forked prompt attachments losing file parts (#17815)

packages/opencode/test/cli/cmd/tui/prompt-part.test.ts

@@ -0,0 +1,47 @@
+import { describe, expect, test } from "bun:test"
+import type { PromptInfo } from "../../../../src/cli/cmd/tui/component/prompt/history"
+import { assign, strip } from "../../../../src/cli/cmd/tui/component/prompt/part"
+
+describe("prompt part", () => {
+  test("strip removes persisted ids from reused file parts", () => {
+    const part = {
+      id: "prt_old",
+      sessionID: "ses_old",
+      messageID: "msg_old",
+      type: "file" as const,
+      mime: "image/png",
+      filename: "tiny.png",
+      url: "data:image/png;base64,abc",
+    }
+
+    expect(strip(part)).toEqual({
+      type: "file",
+      mime: "image/png",
+      filename: "tiny.png",
+      url: "data:image/png;base64,abc",
+    })
+  })
+
+  test("assign overwrites stale runtime ids", () => {
+    const part = {
+      id: "prt_old",
+      sessionID: "ses_old",
+      messageID: "msg_old",
+      type: "file" as const,
+      mime: "image/png",
+      filename: "tiny.png",
+      url: "data:image/png;base64,abc",
+    } as PromptInfo["parts"][number]
+
+    const next = assign(part)
+
+    expect(next.id).not.toBe("prt_old")
+    expect(next.id.startsWith("prt_")).toBe(true)
+    expect(next).toMatchObject({
+      type: "file",
+      mime: "image/png",
+      filename: "tiny.png",
+      url: "data:image/png;base64,abc",
+    })
+  })
+})