fix: allow custodian to freeze full balance, add edge tests, and clean unused code

[ruhulamin1398]

Summary

This PR improves the ERC20Custodian contract by:

• Fixing the freeze function to allow freezing up to the user's total balance rather than just the available (unfrozen) balance.
• Adding edge test cases covering full balance freeze, zero freeze, and updates to frozen amounts.
• Removing unused code: the TokensUnfrozen event and ERC20InsufficientFrozenBalance error which were declared but never used.
• Updating NatSpec comments in the contract for clarity.
• Updating the documentation Markdown file to reflect the revised behavior.

Changes

• ✅ Updated freeze() to check against balanceOf() instead of availableBalance()
• ✅ Added edge test cases for freezing entire balance, freezing zero tokens, and modifying frozen balances
• ✅ Removed unused TokensUnfrozen event and ERC20InsufficientFrozenBalance error
• ✅ Improved doc comments in the Solidity contract
• ✅ Updated docs/token/ERC20/extensions/ERC20Custodian.md to match code changes