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/base.py

@@ -382,7 +382,7 @@ class BaseAgent(ABC):
         when the conversation was compressed after that turn was produced —
         the compressed local history is the context then, not the server's.
         """
-        if not getattr(settings, "OPENAI_RESPONSES_CHAIN_ACROSS_TURNS", True):
+        if not settings.OPENAI_RESPONSES_CHAIN_ACROSS_TURNS:
             return None
         if not self.chat_history:
             return None
@@ -422,7 +422,7 @@ class BaseAgent(ABC):
             # No provider-reported usage on the previous turn (older rows,
             # estimate-only providers): nothing to bound against.
             return meta["response_id"]
-        budget = getattr(settings, "OPENAI_RESPONSES_CHAIN_BUDGET_TOKENS", None)
+        budget = settings.OPENAI_RESPONSES_CHAIN_BUDGET_TOKENS
         if not budget:
             from docsgpt.core.model_utils import get_token_limit