make enthalpy_units required in Python API

[markleader]

Summary

Finalize the Python Reactant.enthalpy_units migration by removing the implicit legacy fallback when enthalpy is provided without units. Closes #65.

This makes enthalpy_units required whenever Reactant.enthalpy is passed, updates the typed Python surface and docs to match, and replaces the transitional warning-based regression with the new error behavior.

Changes

• Update cea.Reactant in the Python binding to raise ValueError when enthalpy is provided without enthalpy_units
• Keep explicit-unit handling unchanged for supported molar and weight-based unit spellings
• Preserve the existing molecular_weight requirement for weight-based enthalpy units and the no-molecular_weight path for molar units
• Update the Python type stubs so overloads reflect the required-units contract
• Replace the warning regression test with a required-units failure regression
• Update Python docs and the RP-1311 Example 5 docs to describe the finalized required-units behavior
• Update CHANGELOG.md to record the breaking Python API change

Testing

Compatibility / Numerical behavior

• No expected changes to numerical results
• Expected changes (explain and provide validation)

This is an intentional Python API break: Reactant(enthalpy=...) without enthalpy_units=... now raises ValueError instead of falling back to implicit J/kg.

For callers already providing explicit enthalpy units, no numerical behavior is intended to change.