Fix Mixture.calc_property inconsistent errors for invalid prop_type

[djkees]

Summary

Fixes an inconsistency where Mixture.calc_property raised different, unrelated exception types depending on the sign of an invalid prop_type: a positive out-of-range value correctly raised ValueError, but a negative value raised OverflowError with a Cython-internal message, bypassing the method's own validation entirely.

Changes

• source/bind/python/CEA.pyx: Mixture.calc_property's prop_type parameter changed from the Cython enum type cea_property_type to a plain int. This removes Cython's automatic enum-bounds coercion at the function boundary (the actual source of the stray OverflowError), so every invalid value — positive or negative — now falls through uniformly to the method's existing if prop_type not in [...]: raise ValueError(...) allowlist check.
• The four downstream calls into the C ABI (cea_mixture_calc_property, cea_mixture_calc_property_tp, and their _multitemp variants) still require the cea_property_type C enum per their fixed signatures in cea_def.pxd, so each call site now has an explicit <cea_property_type>prop_type cast. This is a compile-time C-level cast with no runtime coercion, and by the time it runs prop_type has already passed the allowlist check, so it never sees an invalid value.
• source/bind/python/tests/test_mixture.py: test_calc_property_rejects_unknown_type is now parametrized over both 999999 (pre-existing positive case) and -5, asserting ValueError for both.

Testing

• make py-rebuild
• pytest source/bind/python/tests -v — 114 passed
• Manually reproduced the reported repro steps; both now raise ValueError: Property type not supported for mixture calculations instead of one raising OverflowError

Compatibility / Numerical behavior

• No expected changes to numerical results

This only changes exception-raising behavior for already-invalid prop_type inputs; no valid-input code path or numerical calculation changes.

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Drafted with Claude's assistance

• Verified empirically by rebuilding the binding and running the full Python test suite, plus a manual repro of both exception paths
• Verified by reading CEA.pyx's calc_property method and cea_def.pxd's C-call signatures directly to trace why the enum-typed parameter caused the coercion, and why the downstream calls needed explicit casts

🤖 Generated with Claude Code

[djkees]

Superseded by #163, which combines this fix with #162's into one PR since both touch the same code region.