Skip untestable files in new_pr_handler and close PR

[hiroshinishio]

Summary

• When the dashboard or schedule triggers a coverage PR for a file with no testable code (e.g. __init__.py with only a docstring), the agent now checks should_skip_test() before proceeding
• If the file has no testable code, the PR is closed with an explanation instead of wasting agent tokens on meaningless tests
• Also added explicit handling for single-line module docstrings in should_skip_python

Root cause

The dashboard allows users to select any file from the coverage table for test generation. Files like __init__.py that only contain a docstring pass is_source_file() (because .py extension) and enter the coverage DB. The agent then writes trivial tests like test_package_is_importable that add no value.

Related finding

GA's restrict_edit_to_target_test_file_only=True policy prevents it from fixing implementation bugs it discovers. In joeHeld/Unit-Testing-Demo, GA found 4 bugs in logic.py but couldn't fix them - so it wrote tests matching the buggy behavior instead. Worth discussing whether coverage PRs should allow impl fixes when bugs are found.