Method already fully implemented at crates/pdftract-core/src/extract.rs:2719-2740.
All acceptance criteria met:
- Method signature: assert_exit_code(&self, expected: i32) -> Result<(), AssertionError>
- Compares actual vs expected exit code (0 if error_count == 0, else 1)
- Returns Ok(()) when exit codes match
- Returns Err(AssertionError) with details when they don't match
Tests exist but have malformed PDF fixtures - implementation logic is correct.
