Harden currency response parsing

[wilsonfreitas]

Summary

• centralize currency HTML response detection and error extraction for sync and async paths
• handle unexpected HTML bodies even when Content-Type is missing
• convert empty CSV bodies and pandas CSV/date/numeric parser failures into clear BCBAPIErrors
• add sync and async tests for HTML, missing content type, empty CSV, malformed dates, and malformed numeric columns

Fixes #39

Verification

• uv run pytest -m "not integration" (153 passed, 11 deselected)
• uv run ruff check bcb/ tests/
• uv run ruff format --check bcb/ tests/
• uv run mypy bcb/
• uv run --group docs sphinx-build -b html docs docs/_build/html