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.

docsgpt/graphrag/store.py

@@ -77,11 +77,9 @@ class GraphStore:
     """Stores and queries a per-source knowledge graph in the pgvector DB."""
 
     def __init__(self, connection_string: Optional[str] = None):
-        self._connection_string = connection_string or getattr(
-            settings, "PGVECTOR_CONNECTION_STRING", None
-        )
+        self._connection_string = connection_string or settings.PGVECTOR_CONNECTION_STRING
 
-        if not self._connection_string and getattr(settings, "POSTGRES_URI", None):
+        if not self._connection_string and settings.POSTGRES_URI:
             from docsgpt.core.db_uri import normalize_pgvector_connection_string
 
             self._connection_string = normalize_pgvector_connection_string(