assert finalized contracts reject all mutating entrypoints

[HademiData]

Closes #571

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Summary

Adds security tests verifying finalized escrow contracts remain immutable.

Changes

• Added tests ensuring finalized contracts reject mutating operations.
• Added test verifying a second finalization attempt returns AlreadyFinalized.
• Added tests confirming read-only endpoints (get_contract and get_finalization_record) continue to work after finalization.
• Enabled the new security test module in the test suite.

Validation

• cargo fmt
• cargo build
• cargo test

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@HademiData Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

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[mikewheeleer]

great stuff — thanks for picking it up 🙌