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/agents/workflow_agent.py

@@ -331,7 +331,7 @@ class WorkflowAgent(BaseAgent):
         from docsgpt.storage.storage_creator import StorageCreator
 
         storage = StorageCreator.get_storage()
-        max_bytes = int(getattr(settings, "ARTIFACT_MAX_BYTES", 0) or 0)
+        max_bytes = int(settings.ARTIFACT_MAX_BYTES or 0)
         dropped: List[str] = []
         if len(self.attachments) > _MAX_INPUT_DOCUMENTS:
             over = len(self.attachments) - _MAX_INPUT_DOCUMENTS