GraalPy: bytearray for the c format is not refused (test_fields_bytearray_pack_2004) #2015

Describe the bug
On GraalPy, TestBytearrayPack.test_bytearray_for_the_c_format_stays_refused in tests/corekit/test_fields_bytearray_pack_2004_unit.py fails: packing a bytearray for the c format does not raise. The test pins CPython's struct behaviour, which GraalPy's struct does not share.

Reproduction
The graalpy-25.4 experimental leg on main f96a6b156 (job):

FAILED tests/corekit/test_fields_bytearray_pack_2004_unit.py::TestBytearrayPack::test_bytearray_for_the_c_format_stays_refused - AssertionError: (<class '_struct.StructError'>, <class 'TypeError'>) not raised

Expected behavior
The test should pin what Field.pack does, so it holds on every engine, and Field.pack should refuse or accept a bytearray for c consistently across engines.

System information

• OS Version: ubuntu-latest (GitHub Actions)
• Python Implementation: GraalPy 25.4
• commit: f96a6b156

Additional context
This came out of the #1963 re-measure, now that the leg runs to completion in shards (#2012). The test was added by #2007, for #2004.