Add (string-matching based) constexpr support to functions and methods

[jbcoe]

No description provided.

[Copilot]

Pull request overview

Adds basic constexpr awareness to the C++ model layer by detecting constexpr on parsed functions/methods and reflecting it in their string representations, with corresponding test updates.

Changes:

• Extend _Function to detect constexpr via token inspection and include it in formatting.
• Introduce __str__ for _Function, Function, and Method (and align __repr__ to it).
• Update tests/test_cppmodel.py to include and assert constexpr function/method rendering.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
tests/test_cppmodel.py	Adds constexpr declarations to the sample source and asserts they appear in the model output.
src/xyz/cppmodel.py	Detects constexpr and prepends it in function/method string formatting; adds __str__ implementations.

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[nbx8]

Looks good to me.