contract(round): document and test all custom error codes

[karagozemin]

Summary

Ensure every custom error in the round contract is documented in contracts/round/ERRORS.md and has a test that triggers it, with a drift check keeping docs and code in sync.

Motivation

Downstream SDK error mapping (contractErrorCode) depends on stable, documented codes; undocumented codes cause opaque failures for integrators.

Acceptance criteria

• ERRORS.md lists every error variant with meaning and trigger.
• A test exercises each error path.
• Drift check fails if a code exists in code but not docs (or vice versa).
• Runs in CI.

[ifygreg01-best]

I have an experience with this issue
I would love to work on it, already reviewed the issue please assign me on this

[blvckmamba81-sketch]

Hi! I'd like to work on this issue. I have experience with Solidity smart contracts and testing, and I can document all custom error codes while adding comprehensive tests to ensure they're properly covered.

[Heazzy500]

Hello, I’ve gone through the requirements and I’m confident I can implement a proper solution that matches the acceptance criteria. Kindly assign this issue to me.

[Joyful12-tech]

Hello Maintainer,

I'd like to contribute to this issue. I've reviewed the requirements and understand the importance of keeping the contract's error definitions, documentation, and test coverage in sync to ensure reliable SDK integrations. If assigned, I'll document each error variant, add comprehensive test coverage for every error path, implement the required drift check, and ensure the solution integrates cleanly with the existing CI workflow.
Thank you for your time and consideration. I look forward to contributing to the project.