Replace hardcoded "Sec-WebSocket-Version" with constant

[kwondh5217]

Motivation

The current implementation uses a hardcoded string "Sec-WebSocket-Version" in the method, while also using the constant WebSocketHttpHeaders.SEC_WEBSOCKET_VERSION within the same method. This inconsistency could lead to potential errors during maintenance and reduces code readability.

Modification

• Replaced the hardcoded string "Sec-WebSocket-Version" with the constant WebSocketHttpHeaders.SEC_WEBSOCKET_VERSION.
• Ensures consistent usage of constants throughout the method for better readability and maintainability.

Result

This is a purely refactor-level change. No functional behavior has been altered, and existing functionality remains intact.