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/tool_executor.py

@@ -53,7 +53,7 @@ def _dedupable_tool_names() -> frozenset:
     """
     from docsgpt.core.settings import settings
 
-    return frozenset(BUILTIN_AGENT_TOOLS) | frozenset(getattr(settings, "DEFAULT_CHAT_TOOLS", None) or [])
+    return frozenset(BUILTIN_AGENT_TOOLS) | frozenset(settings.DEFAULT_CHAT_TOOLS or [])
 
 
 def _requires_approval(tool: Dict, action: Dict) -> bool: