Fix/payload buffer reserve

[andinux]

This PR fixes the payload encoder’s buffer bookkeeping and adds coverage to catch regressions:

• adjust cloudsync_buffer_check so the first row allocates space for the payload header before reusing or growing the working buffer
• add unit-level “Payload Buffer” scenarios that replicate the Android test sequence and verify encoded length for blobs up to 10 MB, making the payload helper size configurable