[Test Proxy] Update environment_variables scope (Azure#26076)

This updates the environment_variables pytest fixture
scope to "session" (as opposed to the default "function" scope)
in order to enable users to use it in other session scoped
fixtures. Also, for most cases users will only need
one instance of EnvironmentVariableSanitizer per test session since
the envars won't typically change during the test run.

Signed-off-by: Paul Van Eck <[email protected]>

tools/azure-sdk-tools/devtools_testutils/proxy_fixtures.py

@@ -94,10 +94,13 @@ def get_or_record(self, variable: str, default: str) -> str:
         return self.variables.setdefault(variable, default)
 
 
-@pytest.fixture
+@pytest.fixture(scope="session")
 def environment_variables(test_proxy: None) -> EnvironmentVariableSanitizer:
     """Fixture that returns an EnvironmentVariableSanitizer for convenient environment variable fetching and sanitizing.
 
+    This fixture is session-scoped, so a single instance of EnvironmentVariableSanitizer is shared across all
+    tests using this fixture in the test session.
+
     :param test_proxy: The fixture responsible for starting up the test proxy server.
     :type test_proxy: None