api_version tests fail on 5.7.0 after the custom code was reverted #2745

Confirm this is a Python library issue and not an underlying Cloudflare API issue.

• This is an issue with the Python library

Describe the bug

Six tests in tests/test_client.py fail on a clean checkout of v5.7.0:

FAILED tests/test_client.py::TestCloudflare::test_api_version_env_var
FAILED tests/test_client.py::TestCloudflare::test_api_version_omitted_when_empty
FAILED tests/test_client.py::TestCloudflare::test_api_version_rejects_invalid_format
FAILED tests/test_client.py::TestAsyncCloudflare::test_api_version_env_var
FAILED tests/test_client.py::TestAsyncCloudflare::test_api_version_omitted_when_empty
FAILED tests/test_client.py::TestAsyncCloudflare::test_api_version_rejects_invalid_format
6 failed, 152 passed, 2 skipped

The tests expect api_version to fall back to CLOUDFLARE_API_VERSION, an invalid
api_version to raise ValueError, and the API-Version header to be omitted when no
version is configured. Cloudflare.__init__ does none of those. It defaults api_version
to datetime.today().strftime("%Y-%m-%d") and always sends the header, so every request
advertises a value like 2026-09-20, which does not match the <date>.<channel> form the
tests describe.

4e822eb ("feat: add API-Version header support") added the implementation and the tests
together. ecdba20 ("chore: revert api_version custom code") reverted the implementation
in _base_client.py, _client.py and _version.py but left the tests, and they have
failed since.

Part of the feature is still generated: 5618be9 ("Update bundle: brapi") put
__api_version__ = "" back into _version.py after the revert, so codegen still emits the
symbol while the code that read it is gone.

Was the revert intentional? If the API-Version header is not shipping, the tests should
go too. If it is shipping, the implementation needs to come back, and presumably from the
Stainless config rather than as edits to _client.py, since direct edits are what got
reverted. I have a local branch restoring the reverted code and can open a PR either way,
but I would rather know which direction is wanted first.

To Reproduce

1. git clone https://github.com/cloudflare/cloudflare-python && cd cloudflare-python
2. git checkout v5.7.0
3. uv sync --all-extras
4. uv run pytest tests/test_client.py -k api_version

Code snippets

>>> from cloudflare import Cloudflare
>>> from cloudflare._models import FinalRequestOptions
>>> c = Cloudflare(api_token="x")
>>> c._build_request(FinalRequestOptions(method="get", url="/foo")).headers["api-version"]
'2026-09-20'

OS

Linux

Python version

Python 3.14.6

Library version

cloudflare 5.7.0