fix(graph): handle loop body steps in graph traversal

[pocky]

Summary

• Graph traversal was blind to loop steps: ExecutionOrder skipped body steps entirely and NextDefaultStep ignored Loop.OnComplete, causing incorrect step ordering and reachability analysis for for_each/while workflows.
• ExecutionOrder now inlines loop body steps immediately after the loop step before advancing to on_complete, matching actual runtime execution order.
• NextDefaultStep now prefers Loop.OnComplete over OnSuccess as the default continuation target for loop steps, with OnSuccess retained as fallback when on_complete is absent.
• GetTransitions now emits body step names and on_complete as outbound edges for loop steps, so reachability analysis (FindReachableStates) correctly marks body steps and post-loop targets as reachable.

Changes

Graph Traversal

• internal/domain/workflow/graph.go: Extend NextDefaultStep to check Loop.OnComplete before OnSuccess; extend ExecutionOrder to inline loop body steps; extend GetTransitions to include loop body references and on_complete as outbound edges.

Tests

• internal/domain/workflow/graph_test.go: Add 7 tests covering GetTransitions for for_each and while steps, NextDefaultStep with and without on_complete, ExecutionOrder for both loop types with body steps, and FindReachableStates reachability through loop bodies and on_complete.

Test plan

• Run make test-unit and confirm all new graph tests pass with zero failures
• Run make test-race to verify no data races in graph traversal under concurrent access
• Validate a for_each workflow with body steps: awf validate <workflow> should report no unreachable steps
• Run awf run <for_each_workflow> and confirm body steps execute in the declared order before advancing to on_complete

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