Fix test_useragent_header leaking agent env vars

Clear all KNOWN_AGENTS env vars in test_useragent_header before
connecting, mirroring the pattern in test_agent_detection.py. Without
this, running the test inside an AI-agent shell (e.g. Claude Code, which
sets CLAUDECODE=1) causes session.py to append " agent/<product>" to
the User-Agent and the strict `in` assertion fails. CI runners don't
set agent env vars so this passes there, but the test is environment-
sensitive locally.

Co-authored-by: Isaac
Signed-off-by: Vikrant Puppala <vikrant.puppala@databricks.com>

Author: vikrantpuppala

tests/unit/test_session.py

@@ -8,6 +8,7 @@
     THandleIdentifier,
 )
 from databricks.sql.backend.types import SessionId, BackendType
+from databricks.sql.common.agent import KNOWN_AGENTS
 from databricks.sql.session import Session
 
 import databricks.sql
@@ -97,7 +98,9 @@ def test_tls_arg_passthrough(self, mock_client_class, mock_http_client):
         assert kwargs["_tls_client_cert_key_password"] == "key password"
 
     @patch("%s.session.ThriftDatabricksClient" % PACKAGE_NAME)
-    def test_useragent_header(self, mock_client_class):
+    def test_useragent_header(self, mock_client_class, monkeypatch):
+        for env_var, _ in KNOWN_AGENTS:
+            monkeypatch.delenv(env_var, raising=False)
         databricks.sql.connect(**self.DUMMY_CONNECTION_ARGS)
 
         call_kwargs = mock_client_class.call_args[1]