[3.15] Native/contextvar: PyContextVar_* hidden under Py_LIMITED_API

[vlad-scherbich]

Part of #17809

Tier: A
Section: Native / contextvar

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Summary:

src/native/contextvar.rs fails with E0425 on ffi::PyContextVar_New / _Get / _Set when Py_LIMITED_API is on. Those symbols are not in CPython's stable ABI; pyo3-ffi hides them behind not(Py_LIMITED_API).

Local PYO3_USE_ABI3_FORWARD_COMPATIBILITY=1 builds on 3.15 hit this (same developer convention as #18429's test plan). CI 3.15 already compiles without this change. It does not unblock wheel builds and does not lift wrap().

Fix: local PyContextVar_New / _Get / _Set decls in src/native/contextvar.rs.

Implemented in #19903.