[Bug] HttpxRequestAdapter: The silent deprecation of base_url is actually a breaking change

#481 actually introduced a breaking change.

The parameter base_url is still there but no matter what value you provide, it's ignored. We did not pass a value for http_client, and only specified the base_url like this:

request_adapter = HttpxRequestAdapter(
    authentication_provider, base_url="https:/example.com"
)

We run this in a Fabric notebook where the version of the package was not pinned. So the moment it downloaded the new version, it failed with a misleading error.

To fix it, we had to do the following work around:

request_adapter = HttpxRequestAdapter(authentication_provider)
request_adapter.base_url = "https:/example.com"

I think it's best to deprecate this base_url parameter in the constructor if it's not supposed to be used.

Start with HttpxRequestAdapter and inspect how its constructor handles base_url after change #481; compare the reported workaround with the parameter's current behavior. Clarify whether the intended outcome is deprecating the constructor parameter or restoring its effect, then verify the chosen behavior with regression coverage.