feat(events): add WebSocketConnectionContext helper for APIGatewayV2WebSocketEvent (#273)

[ramanathan1504]

This change addresses #273 by adding a session-like, Lambda-safe connection context for API Gateway WebSocket events.

What changed

• Added WebSocketConnectionContext to APIGatewayV2WebSocketEvent in aws-lambda-java-events/src/main/java/com/amazonaws/services/lambda/runtime/events/APIGatewayV2WebSocketEvent.java
• Added convenience accessors:
  • APIGatewayV2WebSocketEvent#getConnectionContext()
  • APIGatewayV2WebSocketEvent.RequestContext#getConnectionContext()
• Added getManagementApiEndpoint() to build https://{domainName}/{stage} for API Gateway Management API usage
• Added null/empty fallback behavior for endpoint parts (domainName, stage)
• Added tests in aws-lambda-java-events/src/test/java/com/amazonaws/services/lambda/runtime/events/APIGatewayV2WebSocketEventTest.java
• Documented usage and null-safe handling in aws-lambda-java-events/README.md
• Added release note entry in aws-lambda-java-events/RELEASE.CHANGELOG.md

Why

AWS Lambda WebSocket events do not provide a native Java WebSocketSession. This helper provides a lightweight session-like object built from event metadata (connectionId, domainName, stage) so handlers can easily pass one context object through their business logic.

Backward compatibility

• Non-breaking: existing event fields and getters remain unchanged.
• Existing code using requestContext.getConnectionId() continues to work.
• New behavior is additive only.

Error handling / fallback

• getConnectionContext() returns null when requestContext or connectionId is missing.
• getManagementApiEndpoint() returns null when domainName or stage is missing/empty.
• No new exceptions introduced.