fix(slack): deliver file-only messages when all media downloads fail

When a Slack message contains only files/audio (no text) and every file
download fails, `resolveSlackMedia` returns null and `rawBody` becomes
empty, causing `prepareSlackMessage` to silently drop the message.

Build a fallback placeholder from the original file names so the agent
still receives the message, matching the pattern already used in
`resolveSlackThreadHistory` for file-only thread entries.

Closes #25064

src/slack/monitor/message-handler/prepare.test.ts

@@ -222,6 +222,23 @@ describe("slack prepareSlackMessage inbound contract", () => {
     expect(prepared).toBeNull();
   });
 
+  it("delivers file-only message with placeholder when media download fails", async () => {
+    // Files without url_private will fail to download, simulating a download
+    // failure.  The message should still be delivered with a fallback
+    // placeholder instead of being silently dropped (#25064).
+    const prepared = await prepareWithDefaultCtx(
+      createSlackMessage({
+        text: "",
+        files: [{ name: "voice.ogg" }, { name: "photo.jpg" }],
+      }),
+    );
+
+    expect(prepared).toBeTruthy();
+    expect(prepared!.ctxPayload.RawBody).toContain("[Slack file:");
+    expect(prepared!.ctxPayload.RawBody).toContain("voice.ogg");
+    expect(prepared!.ctxPayload.RawBody).toContain("photo.jpg");
+  });
+
   it("keeps channel metadata out of GroupSystemPrompt", async () => {
     const slackCtx = createInboundSlackCtx({
       cfg: {

src/slack/monitor/message-handler/prepare.ts

@@ -362,8 +362,21 @@ export async function prepareSlackMessage(params: {
   const mediaPlaceholder = effectiveDirectMedia
     ? effectiveDirectMedia.map((m) => m.placeholder).join(" ")
     : undefined;
+
+  // When files were attached but all downloads failed, create a fallback
+  // placeholder so the message is still delivered to the agent instead of
+  // being silently dropped (#25064).
+  const fileOnlyFallback =
+    !mediaPlaceholder && (message.files?.length ?? 0) > 0
+      ? message
+          .files!.slice(0, 5)
+          .map((f) => f.name ?? "file")
+          .join(", ")
+      : undefined;
+  const fileOnlyPlaceholder = fileOnlyFallback ? `[Slack file: ${fileOnlyFallback}]` : undefined;
+
   const rawBody =
-    [(message.text ?? "").trim(), attachmentContent?.text, mediaPlaceholder]
+    [(message.text ?? "").trim(), attachmentContent?.text, mediaPlaceholder, fileOnlyPlaceholder]
       .filter(Boolean)
       .join("\n") || "";
   if (!rawBody) {