test(cli): expand scan.py coverage from 51% to 96% with 32 new tests

Add packages/decepticon/tests/unit/cli/test_scan_extra.py covering
_git_diff_files (subprocess success/failure/timeout/OSError), _emit_jsonl_event,
_dispatch_scan_via_sdk (missing SDK, happy-path streaming, asyncio timeout),
_load_findings_graph (missing file, env-var workspace, load exception), and
main() end-to-end paths (no-target, instruction-file error, diff-scope fallback,
SDK RuntimeError/generic exception, happy-path, SARIF output, engagement-name/timeout
overrides, non-interactive forwarding).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: NetVar1337