fix(ingestion): preserve documents without headings

[DTran228]

Summary

Fixes #149 — Structural chunker silently drops documents that contain no headings.

Changes

• Add a level-zero fallback when structural chunking finds no Markdown sections.
• Preserve source metadata for plain-text documents without headings.
• Keep large plain-text inputs protected by the existing semantic sub-chunking path.
• Strengthen structural-chunker regression tests.

Testing

• New/updated tests cover the changes.
• Focused command: pytest tests/unit/test_structural_chunker.py -v -m unit — 16 passed.
• Ruff and Black pass for the changed files.
• The full project suite has unrelated existing failures; this change does not touch those modules.

Notes for Reviewers

The repository .venv points to a removed Python 3.11 executable, so the standard make commands cannot start locally. The fix was verified in a clean temporary environment.