🧹 code health improvement: resolve deep nesting in run_rulespec_cases

[edithatogo]

🎯 What: The code health issue addressed was the deep nesting within the run_rulespec_cases loop. Two helper methods _run_single_rulespec_case and _resolve_durable_outputs were extracted to handle specific case processing and output resolution respectively.
💡 Why: Deeply nested structures hinder readability and make tests or further modifications harder to isolate. Reducing this complex block into manageable functions greatly enhances long-term maintainability.
✅ Verification: Validated that no functionality changed by executing the target unit tests (pytest studies/nz-reconciliation/runner/tests). I also passed mypy and the black formatter successfully.
✨ Result: The run_rulespec_cases method is now straightforward and cleanly delegates work, leaving behind readable intent rather than sprawling logic.

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PR created automatically by Jules for task 7624822572331173430 started by @edithatogo

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