fix(whatsapp): use stream mode for media download to fix empty audio buffer

Fixes #4636

## Problem
WhatsApp voice messages (audio/ogg) were being saved as 0-byte files while
image downloads worked correctly. The issue is that Baileys 7.x
`downloadMediaMessage` with "buffer" mode returns empty data for audio
messages.

## Solution
- Changed from "buffer" mode to "stream" mode in `downloadInboundMedia()`
- Manually collect stream chunks and concatenate into buffer
- Added validation to detect and handle empty buffers

## Testing
- Voice messages now download correctly with actual audio data
- Image downloads continue to work as before
- Tested with WhatsApp voice notes (ogg/opus format)

Before fix: 0 bytes
After fix: 11488 bytes (actual audio data)

src/web/inbound/media.ts

@@ -31,15 +31,30 @@ export async function downloadInboundMedia(
     return undefined;
   }
   try {
-    const buffer = (await downloadMediaMessage(
+    // Use stream mode instead of buffer mode to fix audio download issue
+    // where buffer mode returns empty data for audio messages in Baileys 7.x
+    const stream = await downloadMediaMessage(
       msg as WAMessage,
-      "buffer",
+      "stream",
       {},
       {
         reuploadRequest: sock.updateMediaMessage,
         logger: sock.logger,
       },
-    )) as Buffer;
+    );
+
+    const chunks: Buffer[] = [];
+    for await (const chunk of stream) {
+      chunks.push(chunk as Buffer);
+    }
+    const buffer = Buffer.concat(chunks);
+
+    // Validate buffer is not empty
+    if (!buffer || buffer.length === 0) {
+      logVerbose(`downloadMediaMessage returned empty buffer for ${mimetype}`);
+      return undefined;
+    }
+
     return { buffer, mimetype };
   } catch (err) {
     logVerbose(`downloadMediaMessage failed: ${String(err)}`);