feat(engine): support SSESampler and resolve TIME_WAIT socket exhaustion

[xvlet]

• Core Engine: Implemented SSESampler logic and transport layering in stateful.go and single.go.
• Core Engine: Added SO_LINGER=0 socket option and CloseIdleConnections to prevent TIME_WAIT exhaustion during large scale testing.
• Parser: Added SSESampler tag and url parsing in jmx_parser.go.
• Parser: Fixed GraphQL sampler parsing to support JMeter 5.6.3 HTTPSamplerProxy tag format.
• Domain: Added IsSSESampler property to the Sampler struct in plan.go.
• Tests: Updated JMX test plan templates for WebSocket and GraphQL requests.
  7ca4d2