feat: implement mutation testing via cargo-mutants and improve test coverage (issue #483)

[intelliDean]

Description

This PR implements mutation testing via cargo-mutants to ensure high-quality test coverage on critical financial functions in the property-token contract (Issue #483).

Closes #483

Key Accomplishments:

1. Compilation Issues Resolved:

   • Fixed KycTier duplicate enum variants in contracts/identity/lib.rs.
   • Fixed duplicate CircuitBreaker struct definitions in contracts/lib/src/lib.rs.
   • Restored missing staking fields (share_stakes, etc.) and resolved type/function duplication in contracts/property-token/src/lib.rs.
   • Included the test suite in the contract module via include!("tests.rs") to bypass proc-macro file module limitations.

2. Expanded Test Coverage:

   • Added 21 new unit tests in property-token/src/tests.rs covering all previously missed mutation paths for transfers, share issue/redemptions, dividend distributions, withdrawals, and marketplace asks.
   • Refactored assertions to perform precise value checks (such as verifying exact dividend payout amounts) instead of simple > 0 assertions.
   • All 61 unit tests compile cleanly and pass successfully.

3. cargo-mutants Configuration:

   • Configured .cargo/mutants.toml for standard compilation parameters and targeted exclusions.

4. CI/CD Integration:

   • Created a nightly GitHub Actions workflow (.github/workflows/nightly-mutation-test.yml) to run mutation testing daily on a cron schedule automatically.

5. Documentation:

   • Added a Mutation Testing section to docs/testing-guide.md detailing local execution commands, mutants configuration, and nighty CI workflows.

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Verification Results

Unit Tests

cargo test -p property-token --features std

close issue #483

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