fix: do not populate repo level change while removing library (#2740)

In this PR:
- Do not populate repo level change while removing library

Context: the
[log](https://github.com/googleapis/google-cloud-java/actions/runs/8970167105/job/24633084853?pr=10782)
shows that the script went a full generation when a library is removed
from the generation config.

library_generation/test/utils/generation_config_comparator_unit_tests.py

@@ -178,6 +178,21 @@ def test_compare_config_library_addition(self):
         config_change = result.change_to_libraries[ChangeType.LIBRARIES_ADDITION][0]
         self.assertEqual("new_library", config_change.library_name)
 
+    def test_compare_config_library_removal_does_not_have_repo_or_library_level_change(
+        self,
+    ):
+        self.current_config.libraries = []
+        result = compare_config(
+            baseline_config=self.baseline_config,
+            current_config=self.current_config,
+        )
+        self.assertTrue(
+            len(result.change_to_libraries[ChangeType.REPO_LEVEL_CHANGE]) == 0
+        )
+        self.assertTrue(
+            len(result.change_to_libraries[ChangeType.LIBRARY_LEVEL_CHANGE]) == 0
+        )
+
     def test_compare_config_api_shortname_update_without_library_name(self):
         self.current_config.libraries[0].api_shortname = "new_api_shortname"
         result = compare_config(

library_generation/utils/generation_config_comparator.py

@@ -38,8 +38,17 @@ def compare_config(
     :return: a ConfigChange objects.
     """
     diff = defaultdict(list[LibraryChange])
-    baseline_params = __convert_params_to_sorted_list(baseline_config)
-    current_params = __convert_params_to_sorted_list(current_config)
+    # Exclude `libraries` because an empty library list (e.g., by library
+    # removal) will cause this parameter appears in the sorted param list,
+    # which leads to unequal list of parameters.
+    excluded_params = {"libraries"}
+    baseline_params = __convert_params_to_sorted_list(
+        obj=baseline_config, excluded_params=excluded_params
+    )
+    current_params = __convert_params_to_sorted_list(
+        obj=current_config, excluded_params=excluded_params
+    )
+
     for baseline_param, current_param in zip(baseline_params, current_params):
         if baseline_param == current_param:
             continue
@@ -216,7 +225,7 @@ def __compare_gapic_configs(
         diff[ChangeType.GAPIC_ADDITION].append(config_change)
 
 
-def __convert_params_to_sorted_list(obj: Any) -> List[tuple]:
+def __convert_params_to_sorted_list(obj: Any, excluded_params=None) -> List[tuple]:
     """
     Convert the parameter and its value of a given object to a sorted list of
     tuples.
@@ -230,13 +239,17 @@ def __convert_params_to_sorted_list(obj: Any) -> List[tuple]:
 
     Note that built-in params, e.g., __str__, and methods will be skipped.
 
-    :param obj: an object
+    :param obj: an object.
+    :param excluded_params: excluded params.
     :return: a sorted list of tuples.
     """
+    if excluded_params is None:
+        excluded_params = set()
     param_and_values = []
     for param, value in vars(obj).items():
         if (
-            param.startswith("__")  # skip built-in params
+            param in excluded_params
+            or param.startswith("__")  # skip built-in params
             or callable(getattr(obj, param))  # skip methods
             # skip if the type of param is not one of the following types
             # 1. str