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/slack/actions.ts

@@ -5,6 +5,8 @@ import { resolveSlackAccount } from "./accounts.js";
 import { buildSlackBlocksFallbackText } from "./blocks-fallback.js";
 import { validateSlackBlocksArray } from "./blocks-input.js";
 import { createSlackWebClient } from "./client.js";
+import { resolveSlackMedia } from "./monitor/media.js";
+import type { SlackMediaResult } from "./monitor/media.js";
 import { sendMessageSlack } from "./send.js";
 import { resolveSlackBotToken } from "./token.js";
 
@@ -25,6 +27,12 @@ export type SlackMessageSummary = {
     count?: number;
     users?: string[];
   }>;
+  /** File attachments on this message. Present when the message has files. */
+  files?: Array<{
+    id?: string;
+    name?: string;
+    mimetype?: string;
+  }>;
 };
 
 export type SlackPin = {
@@ -271,3 +279,48 @@ export async function listSlackPins(
   const result = await client.pins.list({ channel: channelId });
   return (result.items ?? []) as SlackPin[];
 }
+
+/**
+ * Downloads a Slack file by ID and saves it to the local media store.
+ * Fetches a fresh download URL via files.info to avoid using stale private URLs.
+ * Returns null when the file cannot be found or downloaded.
+ */
+export async function downloadSlackFile(
+  fileId: string,
+  opts: SlackActionClientOpts & { maxBytes: number },
+): Promise<SlackMediaResult | null> {
+  const token = resolveToken(opts.token, opts.accountId);
+  const client = await getClient(opts);
+
+  // Fetch fresh file metadata (includes a current url_private_download).
+  const info = await client.files.info({ file: fileId });
+  const file = info.file as
+    | {
+        id?: string;
+        name?: string;
+        mimetype?: string;
+        url_private?: string;
+        url_private_download?: string;
+      }
+    | undefined;
+
+  if (!file?.url_private_download && !file?.url_private) {
+    return null;
+  }
+
+  const results = await resolveSlackMedia({
+    files: [
+      {
+        id: file.id,
+        name: file.name,
+        mimetype: file.mimetype,
+        url_private: file.url_private,
+        url_private_download: file.url_private_download,
+      },
+    ],
+    token,
+    maxBytes: opts.maxBytes,
+  });
+
+  return results?.[0] ?? null;
+}