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/parser/tokenization.py

@@ -267,7 +267,7 @@ def get_token_counter(embeddings_name: Optional[str] = None) -> TokenCounter:
         A :class:`HuggingFaceCounter` for a model whose tokenizer could be
         loaded, else a :class:`TiktokenCounter`.
     """
-    name = embeddings_name or getattr(settings, "EMBEDDINGS_NAME", None)
+    name = embeddings_name or settings.EMBEDDINGS_NAME
     key = name or "__default__"
     with _cache_lock:
         if key in _cache: