Two issues with run_erc on KiCad 9:
1. kicad-cli returns non-zero exit code when ERC violations exist.
The handler treated this as a command failure and returned early
with success=false, even though valid JSON output was produced.
Fix: check for output file existence instead of exit code.
2. KiCad 9 nests violations under sheets[].violations instead of
(or in addition to) the top-level violations[] array used by
KiCad 8. The handler only read the top-level array, reporting
0 violations on schematics with sub-sheets.
Fix: iterate sheets[] and collect all nested violations.
Both fixes are backward-compatible with KiCad 8.
6 unit tests added covering non-zero exit codes, KiCad 8 top-level
violations, KiCad 9 sheets[] nesting, mixed structures, zero
violations, and missing output files.
