Fix SWE-Lancer default runs directory

[sylvesterkaczmarek]

Summary

• make SWE-Lancer's default runs directory use the existing project-root helper
• avoid constructing a relative path from the module name string
• add regression coverage that requires the default runs path to match the canonical absolute project runs directory

get_default_runs_dir() currently calls Path(__name__).parent / "runs". Since __name__ is a module name such as swelancer.utils.custom_logging, this produces a relative path like swelancer.utils/runs rather than the project's actual runs/ directory.

The same helper is used as the default value for SWELancerEval.runs_dir, so this can place run output and fallback log output in an unintended working-directory-relative location.

This change delegates to the existing swelancer.utils.general.get_runs_dir() helper, which resolves the project root and returns the canonical absolute runs directory.