Two fixes discovered by real end-to-end smoke test against Aliyun docmind:
1. fileUrlObject must be a ReadStream, not a Buffer. The SDK serializes
Buffers as JSON {type:'Buffer',data:[...]} which the API can't read;
a Stream is uploaded as multipart form data. Switched readFile → createReadStream.
2. QueryDocParserStatus returns data.status (not top-level completed/status).
The markdown output URL is in data.outputFormatResult[].outputFileUrl,
not in a separate GetDocParserResult call. Rewrote polling to check
data.status === 'success', then download markdown from the OSS URL.
Also added image download: markdown contains  references;
the client now downloads those images and rewrites paths to local filenames.
Smoke test verified: prime_sieve.pdf → 15s → markdown with image refs downloaded.
AliyunDocmindClient is now production-ready given an AccessKey.
