fix: validate mid-parental height inputs

[pacharanero]

Summary

Recreates and updates the validation contributed by Dr Akanimoh Osutuk (@FibrinLab) in #78 on a clean branch from current live.

• Validate maternal and paternal heights before calculation.
• Reject None, booleans, non-numeric values, and non-finite values with clear ValueError messages.
• Reject heights outside the existing -8 to +8 SDS error limits.
• Validate sex and reference values explicitly.
• Preserve the existing public function signatures and results for valid inputs.
• Add coverage for invalid inputs and accepted boundary values.

The stale Quickstart.ipynb runtime-output changes and the unused absolute maximum-height constant from the original branch are intentionally omitted.

API compatibility

This does not change valid API output. The API server currently computes the same parental-height SDS values and rejects values outside -8 to +8 SDS before calling mid_parental_height_z(). Valid requests therefore reach the same calculation with unchanged output. This package-level validation protects direct Python consumers and provides the same guard at the source.

Attribution

The implementation commit retains FibrinLab <fibrinlab@gmail.com> as its author and includes Marcus Baw as co-author for the current-tree recreation and refinements. The separate documentation attribution is in rcpch/digital-growth-charts-documentation#184.

Supersedes #78.
Closes #53.

Verification

• s/test --running -q: 145151 passed
• Focused mid-parental-height tests: 11 passed
• Ruff fatal-error checks on changed files: passed
• git diff --check: passed