Implementation of get_swap_rate_logic #107

[anihdev]

This PR implements the get_swap_rate_logic and wrote the essential tests ensuring

• Query rate for TokenA to TokenB with 100 TokenA; verify returned rate matches expected value from pool reserves.
• Edge Case: Pass invalid tokenIn or tokenOut (e.g., non-existent token); expect revert with "Invalid token".
• Edge Case: Query rate for a pair with zero liquidity; expect revert with "Insufficient liquidity".
• Edge Case: Pass amountIn = 0 (if applicable); expect revert or return a valid rate based on specification.
• Edge Case: Query rate for same token pair (e.g., TokenA to TokenA); expect revert with "Invalid pair".

closes #107

[GoSTEAN]

@anihdev Fix conflicts and resolve ci