Bug: baseUrl property can return None in core and oauth modules

[btoron]

Describe the bug

The baseUrl property validation is inconsistent across async modules. Some modules validate that baseURL is not None, while core/_base.py and oauth.py return it without validation, which can cause cryptic downstream errors.

Modules WITH validation (correct):

• capacity.py:52-55
• statistics.py:43-46

@property
def baseUrl(self) -> str:
    if self._config.baseURL is None:
        raise ValueError("Base URL is not configured")
    return self._config.baseURL

Modules WITHOUT validation (bug):

• core/_base.py:59-60
• oauth.py:32-33

@property
def baseUrl(self) -> str:
    return self._config.baseURL  # Can return None!

To Reproduce

1. Create an AsyncOFSC instance with a configuration that results in baseURL = None
2. Call any core or oauth method
3. urljoin(None, "/rest/...") produces a cryptic error instead of a clear "Base URL not configured" message

Expected behavior

All modules should validate baseUrl and raise a clear ValueError("Base URL is not configured") when it's None.

Additional context

This will be automatically fixed if #148 (AsyncClientBase extraction) is implemented, since the validation will live in a single place. However, this is a standalone bug that could be fixed immediately with a 2-line change in each affected file.