Refactor `require` statements to use Solidity Custom Errors for gas optimization

[Atharva0506]

Description

The VouchMe smart contract currently relies on require(..., "String") statements for error handling. Upgrading these to Solidity Custom Errors (e.g., error TestimonialDoesNotExist();) will noticeably reduce both deployment size and execution gas costs, as custom errors are much cheaper than storing and reverting with string messages.
Currently, there are 6 instances of require statements with string messages in contracts/src/VouchMe.sol:

• "Invalid signature"
• "Testimonial does not exist"
• "Testimonial has been deleted"
• "Tokens are non-transferrable"
• "Only recipient can delete"
• "Testimonial already deleted"

Proposed Solution

Refactor the contract to use Custom Errors instead of require statements with string messages.

1. Define custom errors at the contract level (or in a shared interface/library), for example:

   error InvalidSignature();
   error TestimonialDoesNotExist();
   error TestimonialAlreadyDeleted();
   error TokensAreNonTransferrable();
   error OnlyRecipientCanDelete();

2. Replace the existing require(condition, "message") logic with standard if/revert patterns:

   if (!condition) {
       revert CustomErrorName();
   }

3. Update the corresponding tests in the Hardhat/Foundry test suite to expect the new custom errors instead of the old revert strings.

Benefits

• Gas Optimization: Reverting with a custom error is cheaper than reverting with a string.
• Smaller Deployment Size: Removing large revert strings keeps the contract bytecode smaller.
• Better Developer Experience: Custom errors appear explicitly in the ABI, making it easier for frontends and indexers to properly catch and handle specific revert reasons.

[Atharva0506]

@Zahnentferner @KanishkSogani does this look good to you? If so could you assign it to me so I can go ahead and open a PR?