[bsergean]

When a server rejects a WebSocket handshake, applications receive its HTTP status and reason but cannot inspect response headers such as Retry-After or custom authentication details.

Add WebSocketErrorInfo::headers and populate it from the existing WebSocketInitResult headers when reporting a connection error. Document access through msg->errorInfo.headers. This is a small C++11-compatible API addition; header parsing is unchanged.

Closes #636.

Validation:

• End-to-end tests reject real WebSocket handshakes with HTTP 401 and 503, then verify the Error callback receives the status, reason, custom headers, and Retry-After with case-insensitive lookup.
• Full local OpenSSL suite: 25/25 passing.
• HTTP header tests pass under ThreadSanitizer (52 assertions across 5 test cases).
• Formatting and git diff --check pass.