feat(slack): add download-file action for on-demand file attachment access (#24723)

* feat(slack): add download-file action for on-demand file attachment access

Adds a new `download-file` message tool action that allows the agent to
download Slack file attachments by file ID on demand. This is a prerequisite
for accessing images posted in thread history, where file attachments are
not automatically resolved.

Changes:
- Add `files` field to `SlackMessageSummary` type so file IDs are
  visible in message read results
- Add `downloadSlackFile()` to fetch a file by ID via `files.info`
  and resolve it through the existing `resolveSlackMedia()` pipeline
- Register `download-file` in `CHANNEL_MESSAGE_ACTION_NAMES`,
  `MESSAGE_ACTION_TARGET_MODE`, and `listSlackMessageActions`
- Add `downloadFile` dispatch case in `handleSlackAction`
- Wire agent-facing `download-file` → internal `downloadFile` in
  `handleSlackMessageAction`

Closes #24681

* style: fix formatting in slack-actions and actions

* test(slack): cover download-file action path

---------

Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>

src/agents/tools/slack-actions.test.ts

@@ -3,6 +3,7 @@ import type { OpenClawConfig } from "../../config/config.js";
 import { handleSlackAction } from "./slack-actions.js";
 
 const deleteSlackMessage = vi.fn(async (..._args: unknown[]) => ({}));
+const downloadSlackFile = vi.fn(async (..._args: unknown[]) => null);
 const editSlackMessage = vi.fn(async (..._args: unknown[]) => ({}));
 const getSlackMemberInfo = vi.fn(async (..._args: unknown[]) => ({}));
 const listSlackEmojis = vi.fn(async (..._args: unknown[]) => ({}));
@@ -19,6 +20,7 @@ const unpinSlackMessage = vi.fn(async (..._args: unknown[]) => ({}));
 vi.mock("../../slack/actions.js", () => ({
   deleteSlackMessage: (...args: Parameters<typeof deleteSlackMessage>) =>
     deleteSlackMessage(...args),
+  downloadSlackFile: (...args: Parameters<typeof downloadSlackFile>) => downloadSlackFile(...args),
   editSlackMessage: (...args: Parameters<typeof editSlackMessage>) => editSlackMessage(...args),
   getSlackMemberInfo: (...args: Parameters<typeof getSlackMemberInfo>) =>
     getSlackMemberInfo(...args),
@@ -194,6 +196,26 @@ describe("handleSlackAction", () => {
     });
   });
 
+  it("returns a friendly error when downloadFile cannot fetch the attachment", async () => {
+    downloadSlackFile.mockResolvedValueOnce(null);
+    const result = await handleSlackAction(
+      {
+        action: "downloadFile",
+        fileId: "F123",
+      },
+      slackConfig(),
+    );
+    expect(downloadSlackFile).toHaveBeenCalledWith(
+      "F123",
+      expect.objectContaining({ maxBytes: 20 * 1024 * 1024 }),
+    );
+    expect(result).toEqual(
+      expect.objectContaining({
+        details: expect.objectContaining({ ok: false }),
+      }),
+    );
+  });
+
   it.each([
     {
       name: "JSON blocks",