Validate public API inputs early

[wilsonfreitas]

Summary

• validate currency side/groupby/output, dates, and symbol lists before HTTP requests
• validate SGS output, last, code inputs, and single-code JSON entry points before HTTP requests
• harden regional economy location/mode validation while preserving state-first handling for ambiguous all-state requests

Fixes #43

Verification

• uv run pytest -m "not integration" (168 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