feat: validate amount/duration upper bounds in create_commitment

[egwujiohaifesinachiperpetual-max]

Closes #469

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This PR #469
addresses security and correctness in the escrow smart contract by adding input validation and overflow prevention for commitment parameters.

🛡️ Security & Correctness Enhancements

1. Upper-Bound Sanity Validation: Enforced boundary limits on commitment parameters to align with backend CommitmentLimits:
   • MAX_AMOUNT: 1_000_000_000_000 (1T tokens)
   • MAX_DURATION_DAYS: 365 (1 year)
   • MAX_PENALTY_BPS: 10_000 (100% basis points early-exit limit)
2. Checked Arithmetic: Replaced simple addition/multiplication in maturity calculation (now + duration * SECONDS_PER_DAY) with safe checked_add and checked_mul to eliminate integer overflow attacks.
3. Structured Errors: Returns InvalidAmount or InvalidDuration if upper bounds or maturity calculations fail validation.

🧪 Robust Test Coverage

• Refactoring: Fixed a nested syntax issue in the test suite to ensure helper tests are declared correctly at the module level.
• New Unit Tests:
  • create_rejects_overflow_duration: Safely triggers maturity multiplication overflows to assert error handling.
  • create_rejects_excessive_amount: Asserts values above MAX_AMOUNT fail.
  • create_rejects_excessive_duration: Asserts durations above MAX_DURATION_DAYS fail.

📚 Documentation

• Added a new Commitment limits section to contracts/README.md to document the constants and error conditions.

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Verification & Build Results

A standalone MinGW/GNU toolchain was configured to successfully compile the smart contract and execute all tests locally:

running 13 tests
.............
test result: ok. 13 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.38s


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### 📂 Walkthrough File Created
I have generated a walkthrough artifact detailing all of the implementation details, verification results, and design choices. You can review it at:  
📄 [walkthrough.md](file:///C:/Users/TOSHIBA/.gemini/antigravity/brain/8a2105ec-79c6-4173-93fe-74726584a49e/walkthrough.md)

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