fix(matrix): handle downloadContent returning {data, contentType} object

[jacoblyles]

Bug

@vector-im/matrix-bot-sdk's downloadContent returns {data: Buffer, contentType: string}, not a raw Buffer. The existing code passed this object directly to Buffer.from(), causing ERR_INVALID_ARG_TYPE for all inbound Matrix media (images, files, audio, video).

Fix

Unwrap the data property from the response before creating the Buffer. Also extracts contentType from the response for proper MIME type detection.

// Before (broken):
const buffer = await params.client.downloadContent(params.mxcUrl);
return { buffer: Buffer.from(buffer) };  // buffer is {data, contentType}, not a Buffer

// After:
const result = await params.client.downloadContent(params.mxcUrl);
const raw = (result as any)?.data ?? result;
const buf = Buffer.isBuffer(raw) ? raw : Buffer.from(raw);
return { buffer: buf, headerType: (result as any)?.contentType };

Testing

Verified on a local Synapse homeserver — images sent from Element iOS now download and are passed to the agent correctly.