Fix pi-extension export/runtime validation and add tests

[falfaddaghi]

Summary

• block pi-extension exports that contain reachable unsupported nodes and exclude them from emitted runtime definitions
• fail exported runtime runs with WF_TRANSITION_INVALID when a requested branch handle is not wired instead of falling back to the first transition
• add direct runtime integration tests plus CLI smoke tests for both failure and happy-path branch execution

Verification

• npm run test:workflow
• npm run test:workflow:cli
• npm test
• npm run typecheck