Fix "code.py" conflict with Python's code module, and fix TestReturnCode_Simple conflict between two test.py files.

[lc0rp]

1 - Rename "code.py" to "sample_code.py" to prevent occasional conflict with Python's code.py module (https://docs.python.org/3/library/code.html). When this conflict occurs, the agent shows errors like "cannot import multiply_int"

2 - Fix TestReturnCode_Simple. There are three "test.py" files: => the one in artifacts_in, which the agent should manipulate => the one in artifacts_out, which is for --mock => the one in custom_python => This one is used to test that everything worked.

The issue is that the third one (custom_python/test.py) will overwrite the first one (artifacts_in/test.py) when both are copied to the workspace. This causes all tests to fail because the output of the original test.py is different from the one for custom_python/test.py (the LLM doesn't write the extra tests because it wasn't asked to do so)

I fixed it by renaming as follows:

• artifacts_in/test.py => artifacts_in/testfile.py
• artifacts_out/test.py => artifacts_out/testfile.py

And changing data.json to reference testfile.py instead of test.py

PR Quality Checklist

• [X] I have run the following commands against my code to ensure it passes our linters:

  black . --exclude test.py
  isort .
  mypy .
  autoflake --remove-all-unused-imports --recursive --ignore-init-module-imports --ignore-pass-after-docstring --in-place agbenchmark