refactor(chart-engine): tighten text-template XML typing

[tmustier]

Summary

Continue chart-engine typing cleanup by removing Any-based XML casts from text_templates.py and replacing them with narrow protocol-based access.

Changes

• clean_slides/chart_engine/text_templates.py
  • removed Any/Union imports and switched to Path | None.
  • introduced local XML protocols:
    • _XmlElementLike
    • _XmlParentLike
  • replaced cast(Any, ...) usage with narrow protocol casts in:
    • load_txbody_template(...)
    • apply_txbody_template(...)
  • tightened apply_txbody_template input type from box: Any to box: object.

Why

• keeps dynamic XML handling explicit and constrained
• improves readability and static typing signal without behavior changes
• aligns with ongoing strict-typing cleanup across chart/render/template code

Validation

• .venv/bin/ruff check clean_slides tests
• .venv/bin/pyright
• .venv/bin/pytest -q
• .venv/bin/pre-commit run --all-files

All green locally.