improve Agent Spec tracing serialization and flow completion reporting

[paul-cayet]

Summary

Fixes WayFlow Agent Spec tracing compatibility issues by ensuring tracing spans/events are serialized with
the WayFlow Agent Spec plugin context, flow executions that end via None transitions still emit a valid
branch_selected, and shared tool-request transform serialization remains compatible with older configs.

Changes

• Serialize Agent Spec tracing events/spans through dump_tracing_model(...) with WayFlow Agent Spec
  plugins.
• Track the last executed flow step internally so flow-end tracing can populate branch_selected even
  without a CompleteStep.
• Consolidate swarm/manager-workers tool request transforms into a shared implementation and register the
  corresponding Agent Spec plugin transform.
• Add legacy upgrade paths for older Agent Spec and WayFlow-native serialized transform names.

[dhilloulinoracle]

Internal regression failed: Build ID #907