refactor: read settings directly instead of getattr with a second default

About 85 call sites read a setting as getattr(settings, "NAME", fallback),
each carrying its own copy of the default. Every one of those names is a
field with a default on the model, so the fallback could never apply to
the real settings object; it only masked drift. Two had drifted:

- OPENAI_PROMPT_CACHE_KEY defaults to True on the model but the reader
  fell back to False, and two test stubs relied on that.
- SharePoint's MICROSOFT_AUTHORITY fallback to
  https://login.microsoftonline.com/<tenant> never fired, because the
  attribute always exists (as None), so MSAL got authority=None. The
  connector now derives the tenant authority when the setting is unset,
  as its test always assumed.

Four places read EMBEDDINGS_KEY straight from os.environ, skipping the
"None"/"" normalisation the model applies; they read the setting now.
Test stubs that replaced a module's settings with a SimpleNamespace list
every setting the code under test reads.

tests/parser/connectors/test_share_point_auth.py

@@ -14,8 +14,8 @@ def mock_settings():
     s.MICROSOFT_TENANT_ID = "tenant-id-123"
     s.CONNECTOR_REDIRECT_BASE_URI = "https://redirect.example.com/callback"
     s.MONGO_DB_NAME = "test_db"
-    # Delete MICROSOFT_AUTHORITY so getattr falls back to default
-    del s.MICROSOFT_AUTHORITY
+    # Unset, as in a real Settings object, so the tenant-derived authority is used.
+    s.MICROSOFT_AUTHORITY = None
     return s