Fix inconsistent Python union creator function naming

[tmimmanuel]

Summary

• Fix Python code generator to use namer_.Function() consistently for union creator function names at both definition and call site (fixes [Python] Inconsistent python generation of union creator types #8843)
• Added dedicated test schema (union_name_test.fbs) with a snake_case union name to expose the naming mismatch, per maintainer feedback on [Python] Inconsistent python generation of union creator types #8843  #8856

Problem

For a union named my_test_union, the generated code produced:

• Definition: def MyTestUnionCreator(...) (via namer_.Function() → UpperCamel)
• Call site: my_test_unionCreator(...) (via namer_.Type() → kKeep)

This caused AttributeError: module has no attribute 'my_test_unionCreator'
at runtime when unpacking unions with non-UpperCamel names.

Fix

One-line change in GenUnPackForUnion in src/idl_gen_python.cpp: replaced
namer_.Type(enum_def) with namer_.Function(enum_def).

Test plan

• New union_name_test.fbs schema with snake_case union name
• New TestUnionCreatorNaming in py_test.py verifies Pack/UnPack round-trip
• Verified test fails without the fix (AttributeError) and passes with it
• CI passes

How to test

cd tests && PYTHONPATH=../python:. python3 -m unittest py_test.TestUnionCreatorNaming -v