Add detector for unused custom error definitions

[ep0chzer0]

Summary

Implements a new detector (unused-error) that identifies custom error definitions that are declared but never used in revert statements.

Fixes #2587

Features

• Detects unused contract-level custom errors
• Detects unused top-level (file-level) custom errors
• Handles errors with and without parameters
• Skips interface contracts (signature-only)

Example

contract VendingMachine {
    error Unauthorized();  // WARNING: declared but never used
    address payable owner = payable(msg.sender);

    function withdraw() public {
        // Missing: if (msg.sender != owner) revert Unauthorized();
        owner.transfer(address(this).balance);
    }
}

The detector will report: Unauthorized is declared but never used in VendingMachine

Test

Added test case in tests/e2e/detectors/test_data/unused-error/0.8.4/unused_custom_error.sol covering:

• Contract-level unused errors (with and without parameters)
• Top-level unused errors
• Used errors (should not be flagged)
• Interface errors (should not be flagged)

Checklist

• Detector implementation
• Test file
• Registered in all_detectors.py
• Added to test suite
• Wiki documentation (to be added after merge)

🤖 Generated with Claude Code

[ep0chzer0]

Hi maintainers, could someone please approve the CI workflow runs when you get a chance? This PR is ready for review. Thanks!

[ep0chzer0]

Note: I noticed there's an existing PR #2565 for similar functionality. However, this PR addresses the review feedback given on #2565:

1. Instance comparison vs name comparison: feat: add detector for unused custom errors #2565 compares errors by name, which can cause false negatives when different contracts define errors with the same name. This PR compares actual CustomError instances using ir.function.custom_error, avoiding name collisions.

2. Individual findings: feat: add detector for unused custom errors #2565 groups all unused errors into a single finding. This PR generates individual findings for each unused error, making it easier to track and fix each one separately.

3. Cleaner output: Each finding includes the contract context (e.g., "Unauthorized is declared but never used in VendingMachine").

Happy to close this if maintainers prefer to continue with #2565, or this could serve as an alternative implementation addressing the review concerns.