[Bug]: Different output upon decoding invalid utf-8

Describe the bug

Not sure whether it is a bug or just a different behaviour.
Not critical either way, but thought filing it for reference anyway.

• When decoding invalid UTF-8 with errors='replace', CPython (and PyPy for that matter) decodes every invalid byte as �.
• GraalPy, OTOH, treats invalid UTF-8 triplets as one character.

Operating system

Linux

CPU architecture

x86_64

GraalPy version

GraalPy 3.12.8 (GraalVM CE Native 25.0.2)

JDK version

No response

Context configuration

No response

Steps to reproduce

GraalPy:

$ graalpy -c "print(b'\xed\xae\x80\xed\xb0\x80'.decode(errors='replace'))"
��

CPython:

$ python -c "print(b'\xed\xae\x80\xed\xb0\x80'.decode(errors='replace'))"
������

Expected behavior

To match CPython unless there is a good reason not to.

Stack trace

Additional context

No response

Start with the reported graalpy -c decoding reproducer and compare its errors='replace' output with CPython for the supplied invalid UTF-8 bytes. Trace the decode entry point to identify where invalid triplets are grouped, then verify that the output matches CPython's replacement behavior.