test: add comprehensive edge-case tests for config, discovery, reporter, and remediation

[Mresyzz]

Summary

This PR addresses high-value test coverage gaps and edge-case boundaries identified during the audit of core modules:

1. config.py: validate_packages, validate_memory_limit, and load_config limits.
2. discovery.py: discover_changed_scripts Git error paths and max_scripts overflow.
3. reporter.py: text sanitization and SARIF rule mappings across all failure states.
4. remediation.py: absolute path interpreter matching.
5. Tracking: updated .ai/v0.9.2_issues_report.md with item 11.

Verification

• Added tests/test_audit_coverage.py with 19 new tests.
• Full pytest suite passes (189 passed, 12 skipped, 0 failures).