feat: expose socket connected state

[Piasy]

Summary

• expose Socket.connected as a public read-only property (private set)
• keep all internal state transitions unchanged
• add a shared connection-state test that validates: before open -> connected -> disconnected
• add a JVM-specific ConnectionTestJvm so the shared connection tests also run on jvmTest

Testing

• run npm ci in kmp-socketio/src/jvmTest/resources
• run ./gradlew :kmp-socketio:linuxX64Test --tests "*ConnectionTestLinux.connectAndEcho*" --tests "*ConnectionTestLinux.shouldExposeConnectionState*"
• run ./gradlew :kmp-socketio:jvmTest --tests "*ConnectionTestJvm.connectAndEcho*" --tests "*ConnectionTestJvm.shouldExposeConnectionState*"

Closes #24

[github-actions]

Overall Project	94.22%	✅
Files changed	100%	✅

File	Coverage
Socket.kt	94.41%	✅

[github-actions]

Overall Project	94.05%	✅
Files changed	100%	✅

File	Coverage
Socket.kt	94.41%	✅