Add support for httpx authentication class

[vruello]

This PR adds support for passing a custom authentication class to the httpx client (#2933) with minimal changes. It is heavily inspired by the existing implementation for the requests client. Internally, it uses the already existing requests_session_auth parameter.

See the corresponding PR in elasticsearch-py : elastic/elastic-transport-python#244

[margaretjgu]

/ai-review

[github-actions]

The code looks mostly correct. A few issues to flag:

Bug: Wrong variable used for httpx auth

In _async/client/__init__.py, the httpx auth block reuses requests_session_auth:

requests_session_auth = http_auth  # <-- wrong variable name
http_auth = DEFAULT

This mirrors the requests pattern but the variable name is semantically wrong. More critically, look at what happens downstream: if requests_session_auth was already set by the requests block above, this overwrites it. And if the consuming code only checks requests_session_auth to pass it to requests-specific node config, the httpx auth value may be silently ignored or misrouted. There should be a separate httpx_session_auth variable (or equivalent) that gets passed to the httpx node config path.

Unused import

In _async/client/utils.py:

from typing import (
    TYPE_CHECKING,
    Any,
)

TYPE_CHECKING is imported but never used.

elastic_transport import may not be available

from elastic_transport import HttpxAsyncHttpNode

If httpx is not installed, HttpxAsyncHttpNode may not be importable from elastic_transport (depending on how elastic_transport handles optional dependencies). This could cause an ImportError at module import time rather than at call time. The is_httpx_http_auth function correctly wraps its import in a try/except, but is_httpx_node_class does not — it imports HttpxAsyncHttpNode unconditionally at module level.

is_httpx_node_class logic gap

The string "httpxasync" is hardcoded. If elastic_transport ever changes the alias, this silently breaks. Minor, but worth a comment referencing where that string is defined.