feat: add property-based tests for payout logic

[EDOHWARES]

Property-Based Testing for Payout Logic

Branch: feature/payout-property-tests
Issue Reference: #329

Description

This PR introduces property-based testing to the PrediFi payout logic using the proptest framework. By generating a wide range of stake and payout scenarios, we ensure that critical invariants of the protocol (such as fee calculation consistency and total stake conservation) are mathematically sound and resistant to edge cases.

Key Changes

• Dependency Update: Added proptest to workspace Cargo.toml and contract dev-dependencies.
• New Test Module: Created src/payout_proptests.rs which verifies:
  • winnings + protocol_fee <= total_stake across all input ranges.
  • Payout stability for winners and non-winners.
  • Consistency of SafeMath share calculations with varying precision.
• Module Registration: Registered the new test module in lib.rs under its own cfg(test) block.

Verification Results

• Ran cargo test payout_proptests with 100+ randomized iterations.
• Confirmed that no arithmetic overflows or rounding errors violate protocol invariants.

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Closes #329

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[EDOHWARES]

@Yunusabdul38 , pls review