test(insurance): add zero/negative amount tests for create_policy (#76)

[tusharshah21]

Summary

Adds four explicit tests to insurance's active test module to ensure create_policy always rejects zero and negative values for monthly_premium and coverage_amount, satisfying issue #76.

Changes

• Added test_create_policy_zero_monthly_premium_panics — asserts monthly_premium == 0 is rejected
• Added test_create_policy_negative_monthly_premium_panics — asserts monthly_premium < 0 is rejected
• Added test_create_policy_zero_coverage_amount_panics — asserts coverage_amount == 0 is rejected
• Added test_create_policy_negative_coverage_amount_panics — asserts coverage_amount < 0 is rejected

Scope

• This PR intentionally avoids refactors, cleanups, and unrelated changes.
• All modifications are limited to what is required by the issue.
• Tests are placed inside the live #[cfg(test)] mod test {} block in lib.rs — the only compiled test module for this crate.

Verification

• cargo test in insurance/ passes all 25 tests (4 new + 21 existing) with 0 failures.

Closes #76

[Baskarayelu]

@tusharshah21 Can you please resolve the conflicts?

[tusharshah21]

done mate