fix(structural): preserve artifacts when parser dependencies are unavailable

[nia-sg-bot]

Summary

• preserve the exact Git snapshot as a schema-valid local artifact when the optional Tree-sitter parser dependency is unavailable
• report the parser installation failure as a file-scoped PARSE_FAILURE instead of aborting or fabricating structural changes
• cover both the structural API and canonical CLI JSON output

Validation

• python3 -m pytest tests/test_structural.py -q
• python3 -m pytest -q
• git diff --check

No schema or CLI contract fields changed; the VS Code extension remains compatible.

Part of #22

Summary by CodeRabbit

• Bug Fixes
  • Structural parser dependency failures are now reported as parse warnings instead of stopping analysis.
  • Analysis can produce a valid artifact even when a file is skipped due to missing structural dependencies.
  • Command-line output now remains successful and includes details about affected files and parse failures.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: c27650db-998d-4117-bcfa-bc785de85f9a

📥 Commits

Reviewing files that changed from the base of the PR and between b100c39 and 855170c.

📒 Files selected for processing (2)

• diffgraph/structural.py
• tests/test_structural.py

Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review.

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Walkthrough

The parser now catches StructuralDependencyError during Python parsing. Structural dependency failures produce PARSE_FAILURE warnings while artifact generation continues with no structural claims.

Changes

Structural parser error handling

Layer / File(s)	Summary
Parse failure handling and validation diffgraph/structural.py, tests/test_structural.py	StructuralDependencyError joins the parser exception handling. Tests verify scoped warnings, empty structural results, valid artifacts, and successful CLI JSON output.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 85517

This localized change preserves valid artifacts when an optional parser is unavailable and reports the failure without changing the schema or CLI contract; no actionable merge-blocking risk remains after normal checks and review.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: preserving artifacts when structural parser dependencies are unavailable.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch nia/issue-22-parser-dependency-warning

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