Implement a robust socket message reader

Implement an interface to read complete messages from a TCP socket where each recv (k) call may return any number of bytes (including 0). Build two variants: (A) length-prefixed messages with a 4-byte big-endian header exposed via readMessage() that returns exactly one message per call, and (B) newline-delimited messages exposed via readLine(). Maintain an internal buffer across calls, correctly handle partial reads, back-to-back messages, and EOF, and provide unit tests and complexity analysis.