Fix test_mode propagation by updating module-level variables

Root cause: When settings.py is first imported, module-level variables like
test_mode are set from sg_config at import time (line 725). Later, when test
fixtures call load_config(test_mode=True), it updates sg_config._test_mode
but the module-level variable stays stale.

The _test_mode property in BaseMixin does:
    from spyglass.settings import test_mode
    return test_mode  # Returns stale module-level variable!

This caused electrode validation to run during tests even though test_mode
should be True, because it was checking the stale module variable that was
set to False at initial import.

Fix: Update module-level variables (test_mode, debug_mode) at the end of
load_config() so they reflect the current config state.

This is why PR #1454's electrode validation worked on master but failed when
merged with our branch - subtle import order differences meant our branch
triggered settings import earlier, before fixtures could update test_mode.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: edeno

src/spyglass/settings.py

@@ -320,6 +320,13 @@ def load_config(
 
         self._set_dj_config_stores()
 
+        # Update module-level variables so they reflect current config state
+        # This is critical for test_mode to work correctly when load_config
+        # is called with test_mode=True after module import
+        global test_mode, debug_mode
+        test_mode = self._test_mode
+        debug_mode = self._debug_mode
+
         return self._config
 
     def _load_env_vars(self) -> dict: