Add vcell_infix_to_python_infix API

[Copilot]

Exposes VCell's Expression.infix_Python() through the native GraalVM layer as a callable Python API, converting VCell expression syntax to Python-compatible infix (e.g. ^ → **, csc(x) → 1.0/math.sin(x)).

Changes

• Python API – vcell_infix_to_python_infix(vcell_infix: str) -> tuple[bool, str, str] added to model_utils.py and exported from libvcell
• Native entrypoint – vcellInfixToPythonInfix @CEntryPoint in Entrypoints.java wraps ModelUtils.get_python_infix() with buffer-size safety and exception handling
• XmlHelper.cloneUsingXML = true – set consistently in all model/solver utility entry methods (was missing, causing cloning failures)
• CI – macOS runners bumped to macos-15 / macos-15-intel

Usage

from libvcell import vcell_infix_to_python_infix

success, msg, python_expr = vcell_infix_to_python_infix("id_1 * csc(id_0 ^ 2.2)")
# python_expr == "(id_1 * (1.0/math.sin(((id_0)**(2.2)))))"

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[jcschaff]

copied description to the correct PR ... closing this one.