test: add report writer artifact tests

[Ayush7614]

Summary

Adds unit tests for pure helpers in strix.report.writer — previously uncovered aside from SARIF tests.

Covers:

• read_run_record() — missing file, corrupt JSON, non-object payload
• write_run_record() round-trip
• render_vulnerability_md() section output
• write_vulnerabilities() — markdown/CSV/JSON artifacts, severity ordering, skip-already-saved IDs
• write_executive_report() output

Why

• Writer helpers are deterministic and easy to regression-test
• Locks in CSV severity sort order and vulnerability artifact layout
• No overlap with open PR test: cover report helpers _parse_dedupe_response and litellm routing predicates #597 (_parse_dedupe_response / usage helpers) or tests/test_sarif.py

Verification

uv run pytest tests/test_report_writer.py -v

Test plan

• pytest tests/test_report_writer.py — 8 passed locally
• ruff check tests/test_report_writer.py — clean

[greptile-apps]

Greptile Summary

This PR adds unit tests for the report writer helpers. The main changes are:

• Run record read and write coverage.
• Vulnerability markdown, CSV, and JSON artifact checks.
• Executive report output coverage.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
tests/test_report_writer.py	Adds focused tests for deterministic report writer behavior, with assertions matching the existing helper contracts.

_{Reviews (1): Last reviewed commit: "test: add report writer artifact tests" | Re-trigger Greptile}

[Ayush7614]

cc: @rajpratham1