🚨 Critical: Add comprehensive unit tests for execution layer

[quantumshiro]

Problem

The execution layer (src/modular/execution.rs) has no dedicated unit tests despite containing 530+ lines of critical transaction processing logic. This violates the CLAUDE.md guidance requiring comprehensive test coverage.

Current Status

• ❌ No unit tests for transaction processing functionality
• ❌ No tests for state management and rollback capabilities
• ❌ No tests for gas metering and resource management
• ❌ No validation of dual transaction model (account-based + eUTXO)

Required Test Coverage

• execute_transaction() method with various transaction types
• execute_block() method with multiple transactions
• execute_contract_transaction() for smart contract deployment/calls
• State management and rollback scenarios
• Gas metering and limit enforcement
• Error handling for invalid transactions
• Integration with smart contract execution engine

Definition of Done

• Create src/modular/execution_tests.rs with comprehensive test suite
• Achieve 90%+ test coverage for execution layer
• All tests pass with cargo test execution --lib
• Tests follow existing patterns from other modular layers

Priority: 🔥 Critical

This is blocking other refactoring work and violates code quality standards.

Estimated Effort: 2-3 days