test: cover issue 1024

[brentthomas248]

Closes #1024

Summary:

• Add focused regression coverage for create_policy returning InsuranceError::InvalidName on empty and overlong names.
• Add focused regression coverage for pay_premium returning InsuranceError::PolicyInactive and InsuranceError::Unauthorized.
• Clean up an existing clippy manual_range_contains warning in insurance/tests/stress_tests.rs.

Verification:

• ~/.cargo/bin/cargo test -p insurance --lib passed in the private build evidence.
• ~/.cargo/bin/cargo clippy -p insurance --all-targets -- -D warnings passed in the private build evidence.
• git diff --check passed.
• Clean-clone git apply --check --unidiff-zero passed against main.

Note:

• The recorded private-build evidence notes that the full cargo test -p insurance suite has existing unrelated stress-test failures, so I kept verification scoped to the passing lib tests and clippy run.