CI: Split Go and contracts tests into separate continuation configs

## Summary

Follow-up from #18812: Split Go tests and contracts tests into separate continuation configs that only run when relevant files change.

## Motivation

Currently, Go tests and contracts tests run on all PRs regardless of what files changed. By creating separate continuation configs (similar to how `kona.yml` only runs on `kona/**` changes), we can:

- Reduce CI time for PRs that only touch contracts (skip Go tests)
- Reduce CI time for PRs that only touch Go code (skip contracts tests)
- Save compute resources

## Proposed Changes

1. Create `.circleci/continue/go-tests.yml` - runs on `**/*.go` changes
2. Create `.circleci/continue/contracts-tests.yml` - runs on `packages/contracts-bedrock/**` changes
3. Update the path-filtering mapping in `config.yml` to route appropriately

## References

- PR comment: https://github.com/ethereum-optimism/optimism/pull/18812#discussion_r2699181261

Start with .circleci/config.yml and the existing .circleci/continue/kona.yml configuration, then compare how path filtering routes changes. Create the separate go-tests.yml and contracts-tests.yml continuations described in the issue and update the mapping so each runs only for its relevant paths. Done means Go-only and contracts-only changes skip the unrelated test suite.

Written by the indexing model from the issue text.