test: dedupe position balance helpers

[liobrasil]

Summary

• centralize Flow collateral and MOET debt position helpers in cadence/tests/test_helpers.cdc
• remove duplicated per-file helper implementations from the rebalance scenario tests
• replace the small hardcoded pid: 1 diagnostic reads in scenarios 3a and 3b with the local pid

Validation

• flow test cadence/tests/rebalance_scenario2_test.cdc cadence/tests/rebalance_scenario3a_test.cdc cadence/tests/rebalance_scenario3b_test.cdc cadence/tests/rebalance_scenario3c_test.cdc cadence/tests/rebalance_scenario3d_test.cdc

Notes

• The touched scenario files still fail at their existing closeYieldVault path with FlowALP minimum-balance assertions. The helper cleanup compiled and executed normally before that pre-existing close step.