test(storage): isolate environment in client tests (#16664)

This PR resolves a unit test failure where the project ID leaked from
the local environment into the client constructor tests, causing
assertions to fail.

**Problem**: Tests like `test_ctor_wo_project`,
`test_ctor_w_custom_endpoint_bypass_auth`, and
`test_ctor_w_custom_endpoint_w_credentials` were asserting that
`client.project` was `None` or a specific project ID (like `"PROJECT"`).
However, they were receiving a project ID set in the user's local
environment (e.g., `"precise-truck-742"`), causing an AssertionError.

**Solution**: Wrapped the client creation in these specific tests using
a

```
with mock.patch.dict("os.environ", {}, clear=True):
```

code block to ensure the environment is clean and isolated during test
execution.

> [!Note]
`os.environ` is not simply a Python `dict`, it is an `os._Environ`
object with special characteristics (i.e. `putenv`, `unsetenv`,
enforcing string-only keys/values, handling case-sensitivity in Windows
OS).
> 
> The approach in this PR ensures that:
> * we don't just move **this** `os.environ` reference to point at an
empty `dict` but rather we replace the existing `dict` _in situ_ so that
any other references to those variables that might be been created
earlier in code execution will now reference the correct values as well.
> * We ensure we retain access to the characteristics of the
`os._Environ` object.
> * Using `clear=True` we explicitly state that we want a clean slate
for this test.

---------

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

Author: chalmerlowe

packages/google-cloud-storage/tests/unit/test_client.py

@@ -300,7 +300,8 @@ def test_ctor_wo_project(self):
         PROJECT = "PROJECT"
         credentials = _make_credentials(project=PROJECT)
 
-        client = self._make_one(credentials=credentials)
+        with mock.patch.dict("os.environ", {}, clear=True):
+            client = self._make_one(credentials=credentials)
 
         self.assertEqual(client.project, PROJECT)
         self.assertIsInstance(client._connection, Connection)
@@ -368,10 +369,11 @@ def test_ctor_w_custom_endpoint_use_auth(self):
 
     def test_ctor_w_custom_endpoint_bypass_auth(self):
         custom_endpoint = "storage-example.p.googleapis.com"
-        client = self._make_one(
-            client_options={"api_endpoint": custom_endpoint},
-            use_auth_w_custom_endpoint=False,
-        )
+        with mock.patch.dict("os.environ", {}, clear=True):
+            client = self._make_one(
+                client_options={"api_endpoint": custom_endpoint},
+                use_auth_w_custom_endpoint=False,
+            )
         self.assertEqual(client._connection.API_BASE_URL, custom_endpoint)
         self.assertEqual(client.project, None)
         self.assertIsInstance(client._connection, Connection)
@@ -381,9 +383,11 @@ def test_ctor_w_custom_endpoint_w_credentials(self):
         PROJECT = "PROJECT"
         custom_endpoint = "storage-example.p.googleapis.com"
         credentials = _make_credentials(project=PROJECT)
-        client = self._make_one(
-            credentials=credentials, client_options={"api_endpoint": custom_endpoint}
-        )
+        with mock.patch.dict("os.environ", {}, clear=True):
+            client = self._make_one(
+                credentials=credentials,
+                client_options={"api_endpoint": custom_endpoint},
+            )
         self.assertEqual(client._connection.API_BASE_URL, custom_endpoint)
         self.assertEqual(client.project, PROJECT)
         self.assertIsInstance(client._connection, Connection)