fix: correctly parse suffix-byte-range in HTTP Range header

[GXbSUtze2j8b]

Fix: suffix-byte-range parsing in build_range_response

Bug: When client sends bytes=-500 (last 500 bytes), code set start=0 and returned bytes 0-500 instead of the last 500 bytes.

Root cause: Suffix-byte-range format bytes=-500 produces parts=['', '500']. The empty parts[0] was treated as "start from 0".

Fix: Added explicit check for empty parts[0] with non-empty parts[1]:

if not parts[0] and len(parts) > 1 and parts[1]:
    start = max(0, file_size - int(parts[1]))
    end = file_size - 1

Ref: #29