test(policies): add panic-free error handling audit

[Lspnjr1]

Summary

• Fix version overflow panic with saturating_add in bump_version
• Add comprehensive panic_free_audit test module with 12 tests covering:
  • Validation error handling (invalid postage, unauthorized delegates)
  • Storage default handling (missing policy, version, rules, tiers, delegate scope)
  • Arithmetic overflow protection (version saturation at u32::MAX)
  • Evaluate edge cases (extreme postage values)
  • Delegate scope removal and sender rule reset
• Document panic-free guarantees in README

Test plan

• Verify all existing tests continue to pass
• Verify new panic_free_audit tests pass
• Confirm contract-facing behavior is documented

Closes Stellar-Mail#1391