Normalize PTAX date parameters

[wilsonfreitas]

Summary

• Normalize PTAX date parameters before OData parameter serialization
• Accept ISO strings, date/datetime, and pandas.Timestamp while preserving existing PTAX-formatted strings
• Document that generalized inputs are converted to the PTAX service format M/D/YYYY

Closes #4

Tests

• uv run ruff format --check bcb tests
• uv run ruff check bcb tests
• uv run mypy bcb
• uv run pytest tests/test_odata.py
• uv run pytest
• uv run --group docs sphinx-build -b html docs docs/_build/html