test(chart-engine): add boundary smoke coverage and trim generator glue

[tmustier]

Summary

Follow-up after chart-engine typing cleanup:

1. add direct module-boundary smoke coverage for chart-engine builder + overlays
2. trim residual compatibility glue in chart_generator.py now that chart-engine modules are strictly typed

Changes

1) Builder/overlay smoke tests

Added tests/test_chart_engine_smoke.py with two end-to-end smoke cases that exercise clean_slides.chart_engine.builder.build_chart(...) directly:

• bar chart with overlays
• waterfall chart with overlays

Each test validates:

• output PPTX is produced
• expected chart type is present
• expected overlay text labels are rendered in shapes

This gives lightweight regression coverage at the chart-engine boundary (without going through CLI wiring).

2) Chart-generator glue simplification

Refactored clean_slides/chart_generator.py to remove now-unnecessary dynamic attribute wrapper plumbing (_require_attr, protocol/callable casts) and use direct imports from typed chart-engine modules.

Behavior remains unchanged; this is a readability/maintainability cleanup.

Validation

• .venv/bin/pyright
• .venv/bin/pytest -q
• .venv/bin/pre-commit run --all-files