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/utils.py

@@ -347,7 +347,7 @@ def generate_agent_image_capability(
     agent_id: object, image_path: object, user_id: object
 ) -> str:
     """Create an HMAC capability for one agent's current internal image."""
-    secret = getattr(settings, "JWT_SECRET_KEY", "")
+    secret = settings.JWT_SECRET_KEY
     if not isinstance(secret, str) or not secret:
         return ""
     try:
@@ -389,7 +389,7 @@ def generate_image_url(image_path, agent_id=None, user_id=None):
     if not capability:
         return ""
     canonical_agent_id = str(uuid.UUID(str(agent_id)))
-    base_url = getattr(settings, "API_URL", "http://localhost:7091").rstrip("/")
+    base_url = settings.API_URL.rstrip("/")
     return f"{base_url}/api/images/{canonical_agent_id}/{capability}"