test(insurance,reporting): coverage_to_premium ratio guard + report t… (#736)

[primexk5]

Description

This PR introduces boundary tests for the coverage_to_premium ratio logic to ensure that nonsensical policies are blocked on creation and that the ratio is accurately reported downstream.

Fixes #736

Changes Included

• Insurance Crate Tests (insurance/tests/ratio_guard.rs):
  • Asserts the on-create ratio boundary: accepts coverage_amount == premium * 12 * 500 but strictly rejects +1 with an UnsupportedCombination error.
  • Verifies the guard correctly interacts with per-CoverageType minimum and maximum bounds (ensuring failures correctly isolate ratio violations vs. type bound violations).
• Reporting Crate Tests (reporting/tests/insurance_report_ratio.rs):
  • Asserts that get_insurance_report.coverage_to_premium_ratio is correctly computed using checked math to prevent division-by-zero errors.
  • Verifies that annual_premium correctly aggregates as monthly_premium * 12.
  • Ensures that deactivated policies are fully excluded from the reporting aggregates and that zero active policies are handled gracefully.

Context & Notes

• Built using Soroban SDK 21.7.7 in a #![no_std] environment.
• Validated via cargo test -p insurance and cargo test -p reporting.

[Baskarayelu]

good call covering the weakest-tested crate - the coverage_to_premium ratio guard plus the report-side assertion close a real gap on insurance. merging 👍