Fix auto-merge premature evaluation by waiting for all check suites t…

[hiroshinishio]

…o complete

• Created get_check_suites() to fetch all check suites for a commit
• Created get_required_status_checks() to get required checks from branch protection
• Updated successful_check_suite_handler to check all suites completed before proceeding
• Use required checks from branch protection if available, otherwise wait for all non-queued suites
• Raise RuntimeError with detailed context instead of silent returns
• Add comprehensive test coverage for new functions