Improve validation for public API inputs

[wilsonfreitas]

Context

Several invalid inputs currently fail late or with indirect errors. Examples include empty SGS code lists, unsupported output values, invalid currency side or groupby, regional economy mode values, and unsupported date inputs.

Scope

Add early validation for public API inputs and align docstrings with accepted types.

Branching

Create the implementation branch from project-health-hardening and merge the fix back into project-health-hardening before any merge to main.

Acceptance Criteria

• Invalid inputs fail early with clear ValueError or project exceptions.
• Empty SGS code lists no longer rely on pandas concat errors.
• Public docstrings and type hints match supported inputs.
• uv run pytest -m "not integration", Ruff, and mypy pass.

[wilsonfreitas]

Completed via #53 and merged into project-health-hardening.