Fix KafkaBaseHook.test_connection missing oauth_cb for managed Kafka

[TimurRakhmatullin86]

Problem

KafkaBaseHook.test_connection() builds its AdminClient from the raw connection
configuration and (since the recent callback-resolution change) resolves dotted-path
callbacks — but it never injects the managed OAuth token callback that get_conn adds
for Google Managed Service for Apache Kafka and Amazon MSK IAM connections. As a result,
"Test Connection" in the UI fails for those managed connections even though the hook
itself authenticates and connects correctly — the test path never gets the oauth_cb
token callback.

def test_connection(self) -> tuple[bool, str]:
    try:
        config = self.get_connection(self.kafka_config_id).extra_dejson
        self._resolve_callbacks(config)          # no managed oauth_cb injection
        t = AdminClient(config).list_topics(timeout=10)

Fix

The configuration-building logic shared with get_conn (bootstrap validation, dotted-path
callback resolution, and the managed OAuth token injection for Google Managed Kafka /
Amazon MSK IAM) is extracted into a _build_config() helper that both get_conn and
test_connection use. The UI test now exercises exactly the same configuration as a real
connection, so managed connections test successfully. get_conn behaviour is unchanged (a
pure extraction).

Tests

• New test_test_connection_injects_managed_kafka_oauth asserts the managed oauth_cb is
  present in the config passed to AdminClient from test_connection.
• The existing test_connection unit tests now use a concrete non-managed broker address
  (localhost:9092) instead of a MagicMock, since test_connection now builds the full
  runtime config and a mock host would spuriously match the managed-Kafka path.
• Full providers/apache/kafka/tests/unit/apache/kafka/hooks/test_base.py suite passes
  (23/23); ruff check and ruff format --check are clean.

Rebased onto main after the dotted-path callback-resolution change landed; the diff was
reworked to route both get_conn and test_connection through the shared _build_config
helper (the earlier revision predated that refactor).