(ethernet_server): Refactor EthernetServer and fix initialization bug

[kaabia]

This PR implements significant refactoring and stabilization for the EthernetServer class to handle hardware limits and TCP state transitions more robustly.

Summary of Improvements:

1. Code Abstraction: The repetitive W5100 4-socket limit check is abstracted into the WIZNET_CHIP_LIMIT macro, reducing redundant code in available(), accept(), write(), and operator bool().
2. Centralized Cleanup: Introduces a private helper, void _socketCleanup(), to handle client disconnections (CLOSE_WAIT) and close out unused CLOSED sockets, making the main public methods cleaner and more focused on their primary duties.
3. Bug Fix (Server Initialization): Corrects EthernetServer::begin() to use Ethernet.socketClose() (instead of socketDisconnect()) if the initial socketListen fails, ensuring the socket is correctly returned to the available pool.
4. Bug Fix (Write Function): Fixes a critical bug in write(const uint8_t*, size_t) where it now correctly accumulates and returns the actual sent_data length, rather than returning the requested buffer size regardless of success.
5. Compilation Fix: Ensures the chip variable is correctly declared in operator bool() to satisfy the requirements of the WIZNET_CHIP_LIMIT macro.