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/api/async_sse.py

@@ -33,7 +33,6 @@ from docsgpt.streaming.async_event_replay import (
 )
 from docsgpt.streaming.async_redis import get_async_redis_instance
 from docsgpt.streaming.event_replay import (
-    DEFAULT_KEEPALIVE_SECONDS,
     DEFAULT_POLL_TIMEOUT_SECONDS,
 )
 from docsgpt.streaming.sse_leases import StreamCapExceeded, acquire_stream_lease
@@ -127,7 +126,7 @@ async def stream_message_events(request: Request) -> Response:
     )
     last_event_id = _normalise_last_event_id(raw_cursor)
     keepalive_seconds = float(
-        getattr(settings, "SSE_KEEPALIVE_SECONDS", DEFAULT_KEEPALIVE_SECONDS)
+        settings.SSE_KEEPALIVE_SECONDS
     )
 
     logger.info(