test: add comprehensive rollback on inner call failure tests (#1539)

[sheyman546]

Closes #1539

Summary

Adds comprehensive rollback and compensation tests that supplement the existing signed-flow coverage by exercising the unsigned and fan-out paths.

Changes

New test mocks

• mock_unsigned_fail_bill::Contract — bill payment step panics; compensation (reverse) methods succeed
• mock_unsigned_fail_insurance::Contract — insurance premium step panics; compensation methods succeed

New tests

Test	What it covers
test_unsigned_rollback_bill_failure_returns_rolled_back	Unsigned flow: bill fails after savings → RemittanceFlowRolledBack
test_unsigned_rollback_insurance_failure_returns_rolled_back	Unsigned flow: insurance fails after savings+bills → RemittanceFlowRolledBack
test_unsigned_rollback_first_step_fails_no_compensation	Unsigned flow: first step fails → CrossContractCallFailed (nothing to comp)
test_fanout_flow_does_not_compensate_on_bill_failure	Fan-out flow: bill fails → CrossContractCallFailed (no compensation applied)

Testing

• All 4 new tests follow the existing patterns (Env::default(), mock_all_auths(), etc.)
• Lock release and nonce management verified on all failure paths
• The existing signed-flow rollback tests pass unchanged

Background

Issue #1539 requested tests for rollback on inner call failure. The signed-flow rollback tests already existed; this PR adds the unsigned and fan-out path coverage to close the gap.