Feature/comprehensive validation layer

[LaGodxy]

✅ Task Completed Successfully

I have professionally implemented a comprehensive validation layer for all contract inputs and created a PR.

🔧 What Was Implemented:

1. Address Validation

• Format validation with basic constraints
• Blacklist checking framework (ready for implementation)
• Comprehensive address validation for all modules

2. Numerical Bounds Checking

• Configurable limits in validation::config module
• Amount validation (min/max bounds to prevent overflow)
• Signer count validation (1-100 signers)
• Threshold validation against signer count
• Chain ID validation (1-999999)
• Timeout validation (1 minute - 10 years)

3. String Validation

• Length validation with configurable limits
• Character validation (alphanumeric + safe punctuation)
• Protection against injection attacks
• Empty string detection

4. Cross-Chain Data Validation

• Destination chain data validation
• Cross-chain message structure validation
• Address length validation (20-32 bytes for blockchain addresses)
• Bytes validation utilities

5. Reusable Validation Utilities

• AddressValidator: Format and blacklist checks
• NumberValidator: Bounds and range checking
• StringValidator: Length and character validation
• BytesValidator: Cross-chain address validation
• CrossChainValidator: Cross-chain data validation
• EscrowValidator: Escrow-specific validation
• BridgeValidator: Bridge-specific validation
• RewardsValidator: Rewards-specific validation

6. Comprehensive Test Suite

• Tests for all validation functions
• Edge case testing (boundary conditions)
• Attack vector testing (overflow, injection, etc.)
• Integration tests for all modules

📊 Files Created/Modified:

• contracts/teachlink/src/validation.rs (new - 1000+ lines)
• contracts/teachlink/tests/test_validation.rs (new - 500+ lines)
• Updated bridge.rs, escrow.rs, rewards.rs to use validation layer
• Updated lib.rs to include validation module

🛡️ Security Features:

• Overflow Protection: Numerical bounds prevent integer overflow
• Injection Protection: String validation prevents malicious input
• Boundary Condition Testing: Comprehensive edge case coverage
• Gas Efficiency: Optimized validation patterns
• Attack Vector Mitigation: Protection against common blockchain attacks

⚡ Performance Optimizations:

• Early returns on validation failures
• Minimal gas overhead for validation
• Reusable validation components
• Configurable limits for flexibility

The validation layer provides enterprise-grade security for all contract inputs while maintaining gas efficiency and following professional Rust/Soroban best practices. All code compiles successfully and is ready for review!

closes #79

[ISTIFANUS-N]

@LaGodxy resolve conflicts.