test: add diagnostic redaction, path normalization, and artifact pairing tests

[dev-khimesh]

Summary

Add 15 deterministic regression tests for diagnostic redaction and artifact pairing.

Changes

• Added tests/test_diagnostic_redaction.py with 15 tests covering:
  • Path normalization: diagnostic metadata paths are repo-relative with forward slashes
  • Redaction: header fields (generated_at, hostname, user, python, platform) do not leak repo paths
  • Redaction: /tmp, /var/tmp, repo checkouts, and hostnames not leaked in diagnostic snapshots
  • Artifact pairing: JSON .logd reference matches actual generated artifact
  • Missing artifact detection: diagnostic_logd_error populated when encryption fails
  • Chunked artifacts: all split parts are referenced in JSON metadata
  • Determinism: commit_id is 8-char hex, paths follow expected naming patterns
  • Report shape: correct JSON structure with pass/fail counts
• All tests deterministic on Windows and Unix-like hosts (using Path, tempfile, forward-slash conventions)

Validation

python3 -m pytest tests/test_diagnostic_redaction.py -v  # 15/15 pass

Closes #1