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/core/model_registry.py

@@ -140,7 +140,7 @@ class ModelRegistry:
         from docsgpt.llm.providers import ALL_PROVIDERS
 
         directories = [BUILTIN_MODELS_DIR]
-        operator_dir = getattr(settings, "MODELS_CONFIG_DIR", None)
+        operator_dir = settings.MODELS_CONFIG_DIR
         if operator_dir:
             op_path = Path(operator_dir)
             if not op_path.exists():

docsgpt/core/settings/connectors.py

@@ -35,7 +35,8 @@ class ConnectorSettings(SettingsGroup):
         default="common", description="Azure AD tenant id, or 'common' for multi-tenant."
     )
     MICROSOFT_AUTHORITY: Optional[str] = Field(
-        default=None, description='Authority URL override, e.g. "https://login.microsoftonline.com/{tenant_id}".'
+        default=None,
+        description="Authority URL override; unset derives https://login.microsoftonline.com/<MICROSOFT_TENANT_ID>.",
     )
 
     # Confluence Cloud integration.