FIX-#7640: Respect AutoSwitchBackend.disable() in init. (#7641)

I introduced a subtle bug that caused some cases of `__init__` to not
respect `AutoSwitchBackend.disable()` here:

https://github.com/modin-project/modin/blob/44ee0178b6ed66553fdc8bc43e4cc9e31c8cf92d/modin/core/storage_formats/pandas/query_compiler_caster.py#L1056

That should be `< 2` as it was before; otherwise `__init__` from a
non-modin input (e.g. the dict in this case) will auto-switch regardless
of the value of `AutoSwitchBackend`.

Resolves #7640

Signed-off-by: sfc-gh-mvashishtha <mahesh.vashishtha@snowflake.com>

Author: sfc-gh-mvashishtha

modin/core/storage_formats/pandas/query_compiler_caster.py

@@ -1050,10 +1050,10 @@ def register_query_compilers(arg):
             input_qc_for_pre_op_switch = input_query_compilers[0]
             input_backend = input_qc_for_pre_op_switch.get_backend()
 
-        # Skip the casting code if there's only one input backend and either
+        # Skip the casting code if there are < 2 input backends and either
         # auto-switching is disabled or the inputs are pinned to the input
         # backend.
-        if len(input_backends) == 1 and (
+        if len(input_backends) < 2 and (
             not AutoSwitchBackend.get() or pin_target_backend is not None
         ):
             f_to_apply = _get_extension_for_method(

modin/tests/pandas/native_df_interoperability/test_compiler_caster.py

@@ -1416,6 +1416,28 @@ def test___init___with_in_memory_data_uses_native_query_compiler(
         ):
             assert data_class(*args, **kwargs).get_backend() == expected_backend
 
+    @pytest.mark.parametrize("data_class", [pd.DataFrame, pd.Series])
+    @backend_test_context(
+        test_backend="Big_Data_Cloud", choices=("Big_Data_Cloud", "Small_Data_Local")
+    )
+    @pytest.mark.parametrize(
+        "auto_switch_backend,expected_backend",
+        [
+            (True, "Small_Data_Local"),
+            (False, "Big_Data_Cloud"),
+        ],
+    )
+    def test_auto_switch_backend_disabled_prevents___init__auto_switch(
+        self, auto_switch_backend, expected_backend, data_class
+    ):
+        register_function_for_pre_op_switch(
+            class_name=data_class.__name__,
+            method="__init__",
+            backend="Big_Data_Cloud",
+        )
+        with config_context(AutoSwitchBackend=auto_switch_backend):
+            assert data_class([1, 2, 3]).get_backend() == expected_backend
+
     @pytest.mark.parametrize(
         "num_input_rows, expected_backend",
         [