Summary

Add support for closing only the output side of a WebSocket connection while continuing to receive data from the server. This enables the "half-close" pattern where a client signals it's done sending but
remains open to receive remaining server messages.

Motivation

Some WebSocket-based applications require the client to signal completion of sending data while still needing to receive responses or acknowledgments from the server. Currently, WebSockex.close/2 initiates a
full close handshake that terminates the entire connection.

The half-close pattern allows:

• Client sends a close frame to indicate "I'm done sending"
• Client continues receiving frames until the server sends its close frame
• Connection terminates cleanly after both sides have closed

Proposed API

# Close output with default close frame
WebSockex.close_output(pid)

# Close output with custom close code and message
WebSockex.close_output(pid, {1000, "Done sending"})

Expected Behavior

• close_output/2 sends a WebSocket close frame but keeps the connection open for receiving
• After calling close_output, send_frame/2 returns {:error, %WebSockex.NotConnectedError{connection_state: :output_closed}}
• The client continues to receive and process incoming frames via callbacks (handle_frame, etc.)
• When the server responds with its close frame, the connection terminates normally
• Callbacks that try to reply with a frame have their replies silently ignored (logged for debugging)

Use Case

Applications where the client streams data to a server, then waits for the server to finish processing and send final results before disconnecting.