gsoc26: Mapping Conversion Layer (Layer 3), #60

[DhanashreePetare]

Pull Request

Description

Implements Layer 3 (cross-class mapping conversion) for the Databus Python Client download pipeline, building on the Layer 2 format handlers (TripleHandler, QuadHandler, TSDHandler) merged in #[link Layer 2 PR number]. Users can convert RDF triples to/from CSV, and convert between RDF triple and quad formats, using --format extended with --graph-name and --base-uri.

Related Issues
Issue #60

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• This change requires a documentation update
• Housekeeping

Checklist:

• My code follows the ruff code style of this project.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
  • poetry run pytest - all tests passed
  • poetry run ruff check - no linting errors

What was added:

• databusclient/filehandling/mapping.py — 5 mapping functions using the Layer 2 handlers as IR (TripleHandler/QuadHandler/TSDHandler). Companion .meta.json preserves RDF datatypes and language tags for lossless CSV round trips. Blank nodes serialized with _: prefix for correct round-trip reconstruction.
• --graph-name and --base-uri flags added to download command.

Tests:

• 5 mapping round trip tests in tests/test_format_round_trips.py (IR captured before conversion, consistent with Layer 2 pattern)
• 29 functional tests in tests/test_mapping_conversions.py covering all 5 directions plus edge cases (blank nodes, missing companion file, missing required flags)

Closes #60

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