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/tools/read_document.py

@@ -255,7 +255,7 @@ class ReadDocumentTool(Tool):
         # The task's per-call time limits are raised to match the awaited window: bound to
         # the base timeout at import, the worker would otherwise self-terminate a large
         # parse long before this await gives up.
-        queue = getattr(settings, "DOCUMENT_PARSE_QUEUE", "parsing")
+        queue = settings.DOCUMENT_PARSE_QUEUE
         try:
             async_result = parse_document.apply_async(
                 args=[artifact_id, parent, self.user_id, options],