tests(bigquery): harden system test teardown and unit test auth isolation (#17964)

Following the merge of PR #17953 for the socket leak fix, this PR
focuses on the remaining orthogonal test and fixture hardening fixes:

1. **System Test Teardown Resilience:** Wrap TagKey and TagValue
resource deletions in `try...except NotFound: pass` during
`TestBigQuery.tearDown()` to prevent cascaded teardown failures when tag
resources were already deleted.
2. **Unit Test ADC Isolation:** Enable `autouse=True` on the
`use_local_magics_context` fixture in `test_magics.py` to prevent
credentials mutation across test runs in uncredentialed CI environments.
3. **InteractiveShell Fixture & Fallback:** Use
`IPython.core.interactiveshell.InteractiveShell` and add fallback unit
test coverage for `magics.Context`.

Related: b/540939659 🦕

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: shuoweil

packages/google-cloud-bigquery/tests/system/test_client.py

@@ -204,12 +204,16 @@ def _still_in_use(bad_request):
             tag_key = key_values.pop()
 
             # Delete tag values first
-            [
-                tag_values_client.delete_tag_value(name=tag_value.name).result()
-                for tag_value in key_values
-            ]
+            for tag_value in key_values:
+                try:
+                    tag_values_client.delete_tag_value(name=tag_value.name).result()
+                except NotFound:
+                    pass
 
-            tag_keys_client.delete_tag_key(name=tag_key.name).result()
+            try:
+                tag_keys_client.delete_tag_key(name=tag_key.name).result()
+            except NotFound:
+                pass
 
     def test_get_service_account_email(self):
         client = Config.CLIENT

packages/google-cloud-bigquery/tests/unit/test_magics.py

@@ -37,13 +37,13 @@
 
 bigquery_storage = pytest.importorskip("google.cloud.bigquery_storage")
 IPython = pytest.importorskip("IPython")
-interactiveshell = pytest.importorskip("IPython.terminal.interactiveshell")
+interactiveshell = pytest.importorskip("IPython.core.interactiveshell")
 tools = pytest.importorskip("IPython.testing.tools")
 io = pytest.importorskip("IPython.utils.io")
 pandas = pytest.importorskip("pandas")
 
 
-@pytest.fixture()
+@pytest.fixture(autouse=True)
 def use_local_magics_context(monkeypatch):
     if magics is not None:  # pragma: NO COVER
         local_context = magics.Context()
@@ -58,8 +58,7 @@ def use_local_magics_context(monkeypatch):
 @pytest.fixture(scope="session")
 def ipython():
     config = tools.default_config()
-    config.TerminalInteractiveShell.simple_prompt = True
-    shell = interactiveshell.TerminalInteractiveShell.instance(config=config)
+    shell = interactiveshell.InteractiveShell.instance(config=config)
     return shell
 
 
@@ -143,10 +142,12 @@ def fail_if(name, globals, locals, fromlist, level):
 }
 
 
-def test_context_with_default_credentials():
-    """When Application Default Credentials are set, the context credentials
-    will be created the first time it is called
+def test_context_resolves_unset_credentials_and_project():
+    """When context credentials and project are unset (None), accessing them
+    resolves them from Application Default Credentials.
     """
+    magics.context._credentials = None
+    magics.context._project = None
     assert magics.context._credentials is None
     assert magics.context._project is None