feat: multiple contract enhancements (#169, #155, #156, #152)

[ayomideadeniran]

Description

This PR implements remediation items from the additional audit report across the Soroban contracts. It consolidates multiple fixes to improve the contract's robustness, correctness, and security.

Changes Included:

• Validate allocation percentages exactly or define remainder behavior: Ensures that token allocation percentages are correctly validated to prevent calculation issues or remainder dust.
• Add slippage parameters for Soroswap swaps: Implements slippage protection for all Soroswap swaps to prevent sandwich attacks and excessive value loss during transactions.
• Add minimum amounts for add_liquidity: Enforces minimum deposit limits when adding liquidity, ensuring that small transactions don't inflate state or cause rounding errors.
• Make withdraw transfer USDC to the user: Correctly redirects the withdrawn USDC directly to the user's wallet address instead of holding it.

Related Issues

Closes #169
Closes #155
Closes #156
Closes #152

Requirements Met

• Updated contracts/src/lib.rs and tests as needed.
• Preserved no_std compatibility.
• Utilized typed errors and checked arithmetic.
• All tests pass, format passes, and clippy has no new warnings.

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