Update Ktor session state on cancel

[masaru87]

Summary

• mark the Ktor WebSocket session inactive when cancel is called
• keep the existing cancellation of the underlying Ktor session
• verify isActive becomes false even when rxFlow was never collected

Previously, isActive was updated only by the rxFlow onCompletion handler. Calling cancel before collecting the Flow disconnected the underlying session but left the public isActive property true.

Testing

• ./gradlew :engine:ktor:jvmTest --no-daemon
• ./gradlew jvmTest --no-daemon