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/vectorstore/pgvector.py

@@ -54,9 +54,9 @@ class PGVectorStore(BaseVectorStore):
         # Use provided connection string or fall back to settings.
         # If PGVECTOR_CONNECTION_STRING is not set but POSTGRES_URI is,
         # reuse the same cluster — normalize from SQLAlchemy dialect to libpq form.
-        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(settings.POSTGRES_URI)