ci: cap build errors in the parser instead of piping into head

[datlechin]

Why

report_build_errors in verify.sh piped xcresult-cases.py errors into head -12. Python ignores SIGPIPE, so a test build with a few hundred errors printed a BrokenPipeError traceback once head closed the pipe. The verdict stayed right, but the output looked like a parser crash. The jq it replaced in #3480 died silently.

Change

• xcresult-cases.py errors takes --limit <n> and stops after n errors.
• verify.sh passes --limit 12 and drops the head.

Verified

• scripts/lib/test_xcresult_cases.py covers the errors output (Broken.swift:4: message), the limit, and a bad limit.
• verify.sh parse on a stub bundle with 1000 build errors prints 12 and no traceback; on main it prints the traceback.

Not in this PR

• The oracle suite in scripts/ci/detect-changed-paths.sh still watches summarize-xcresult.sh but not scripts/lib/xcresult-cases.py.