Function heartbeatFrameFrom

Detects raw websocket heartbeat control frames surfaced as text payloads.

Use when:

• A websocket runtime forwards ping/pong frames through the normal message callback
• The runtime should ignore transport heartbeats instead of treating them as protocol JSON

Expects:

• Raw text payloads such as ping and pong

Returns:

• The heartbeat kind when the text is a control frame, otherwise undefined