fix: retain Python native callback parameter names until registration

[rksharma-owg]

Fixes #1321.

Keep each Python callback parameter's UTF-8 storage alive until jsonnet_native_callback has copied the names. With the pre-3.10 limited API, releasing each temporary bytes object inside the conversion loop leaves dangling pointers: later names can overwrite earlier ones, producing duplicate-binding errors even for distinct parameters.

The regression covers two and three multicharacter parameters and out-of-order named arguments. It fails in all three cases on the unchanged source with a CPython 3.9 build targeting the Python 3.8 stable ABI. After the fix, all 11 Python binding tests pass on both Python 3.9 and Python 3.13 using that same stable-ABI extension. git diff --check also passes.