Export AIRFLOW_TEST_MODE from airflow tasks test without --env-vars (#72291)

* Export AIRFLOW_TEST_MODE from airflow tasks test without --env-vars

The test-mode signal has been hostage to an unrelated flag since it was
introduced in 2020, so Dag code that branches on it never saw it during a
plain `airflow tasks test` run. In Airflow 3 this is the only working
test-mode signal, because the `test_mode` task-context variable is
currently disabled.

* Stop the env-vars test leaking its writes into the pytest session

monkeypatch.delenv records an undo entry only when the key is already set,
so on a clean worker the values task_test writes to the real process
environment survived teardown. Seeding a sentinel instead also tightens the
assertion: the command now has to overwrite a pre-existing value rather than
merely populate an absent one.

---------

Co-authored-by: Eason09053360 <185830721+Eason09053360@users.noreply.github.com>

Author: Eason09053360

airflow-core/src/airflow/cli/commands/task_command.py

@@ -417,7 +417,7 @@ def task_test(args, dag: DAG | None = None) -> None:
     env_vars = {"AIRFLOW_TEST_MODE": "True"}
     if args.env_vars:
         env_vars.update(args.env_vars)
-        os.environ.update(env_vars)
+    os.environ.update(env_vars)
 
     if dag:
         sdk_dag = dag

airflow-core/tests/unit/cli/commands/test_task_command.py

@@ -215,7 +215,18 @@ def test_cli_test_with_params(self):
             )
         )
 
-    def test_cli_test_with_env_vars(self):
+    @pytest.mark.parametrize(
+        ("env_var_args", "expected_foo"),
+        [
+            pytest.param([], "foo=sentinel", id="without-env-vars"),
+            pytest.param(["--env-vars", '{"foo":"bar"}'], "foo=bar", id="with-env-vars"),
+        ],
+    )
+    def test_cli_test_with_env_vars(self, monkeypatch, env_var_args, expected_foo):
+        # setenv (unlike delenv) always records an undo entry, so task_test's writes to the real
+        # process environment cannot leak out; the sentinel proves the command overwrote the key.
+        monkeypatch.setenv("AIRFLOW_TEST_MODE", "sentinel")
+        monkeypatch.setenv("foo", "sentinel")
         with redirect_stdout(io.StringIO()) as stdout:
             task_command.task_test(
                 self.parser.parse_args(
@@ -225,13 +236,12 @@ def test_cli_test_with_env_vars(self):
                         "example_passing_params_via_test_command",
                         "env_var_test_task",
                         DEFAULT_DATE.isoformat(),
-                        "--env-vars",
-                        '{"foo":"bar"}',
+                        *env_var_args,
                     ]
                 )
             )
         output = stdout.getvalue()
-        assert "foo=bar" in output
+        assert expected_foo in output
         assert "AIRFLOW_TEST_MODE=True" in output
 
     @mock.patch(