docs: document running tests when Docker credential helper fails (e.g. WSL)

[rich7420]

Description

Document how to run tests when Docker works and when make test fails (e.g. WSL + Docker Desktop credential helper).

• building-from-source.md: Testing section — make test; Option 1: fix Docker config (backup/clear ~/.docker/config.json); Option 2: run without Docker (unit, envtest, validate). Version reference table (envtest K8s, envtest tool, Go/builder) — use Makefile values, not latest.
• AGENTS.md: Same flow under "Running tests when Docker is unavailable or credential helper fails": fix Docker (WSL) or run without Docker; version note.

Related Issues

Closes #939

Checklist

Note: Ensure your PR title follows the Conventional Commits format (e.g., feat(scheduler): add new feature)

• Self-reviewed
• Added/updated tests (if needed)
• Updated documentation (if needed)

Breaking Changes

Additional Notes