Prevents stale signature values from leaking into the MD5 signing
calculation, matching the news_service.py pattern. Adds a test that
passes a stale signature in extra params and verifies the returned
signature is freshly computed.
Add BoxIMClient with get_credentials and send_p2p_message methods,
using Huihui platform MD5 signing mechanism (sorted keys, 24h timestamp,
12-char alphanumeric nonce). Includes full test coverage.
stefanfeng