refactor: cleanup error checks in Windows

[murfffi]

No description provided.

[gemini-code-assist]

Code Review

This pull request updates the conncheck library by adding a copyright section to the README, improving error handling in the core logic, and expanding test coverage. Specifically, it adds a nil check for SyscallConn results and refactors the Windows-specific peek logic for better error reporting. Feedback suggests further refining the Windows implementation by explicitly checking for socket-related errors like WSAENOTSOCK or WSAENOTCONN to return a more accurate status when setting socket options fails.

[gemini-code-assist]

When sockOptErr is non-nil, the function currently returns StatusUnknown. However, if the error indicates that the socket is closed, not connected, or has been shut down (e.g., WSAENOTSOCK, WSAENOTCONN, or WSAESHUTDOWN), we can definitively return StatusNotOpen. This addresses the TODO on line 66 and improves the accuracy of the liveness check on Windows by reducing false positives (where a closed connection might be assumed open).

if sockOptErr != nil {
	if errors.Is(sockOptErr, windows.WSAENOTSOCK) ||
		errors.Is(sockOptErr, windows.WSAENOTCONN) ||
		errors.Is(sockOptErr, windows.WSAESHUTDOWN) {
		return StatusNotOpen
	}
	return StatusUnknown
}