Make connection endpoint rewiring atomic

[matteomekhail]

Summary

• add an explicit rewire event and connection-manager operation
• preserve the original connection when the new endpoint is missing, incompatible, or cyclic
• avoid deleting the model connection at drag start
• cover successful replacement and invalid-drop rollback

Validation

• ./gradlew :composeApp:jvmTest --tests org.wip.plugintoolkit.features.flows.FlowConnectionRewireTest

Closes #8