Inline DEFAULT_NAMESPACE literal to avoid top-level server.py import

Cosmetic import-graph cleanup on top of #1863's move of `NAMESPACE`
from `namespace.py` into `server/server.py`. Before this change,
`tableauserverclient/__init__.py:2` did `from tableauserverclient.
server.server import NAMESPACE`, which forces `server.server` (and
its transitive endpoint tree) to load just to read a string
constant.

Copilot flagged the circular-import path this creates: `server.
server` -> `server.endpoint.endpoint` -> `from tableauserverclient
import ...` -> `__init__` -> `server.server`. Python resolves the
chain today because the reentrant lookups pull submodules whose
own load doesn't back-reference the partially-initialized parent,
but the shape is fragile and adding a new top-level import to
`__init__.py` before line 2 could easily break it.

Replaced the import with the string literal. Kept in sync with
the `NAMESPACE` constant in `server/server.py`. Also removes one
of the two entry points into `server.server` at package load,
though everything else in `__init__.py` still transitively pulls
the endpoint tree via the models block below, so no real startup
savings -- this is defensive, not a perf fix.

Author: jacalata

tableauserverclient/__init__.py

@@ -1,5 +1,11 @@
 from tableauserverclient.bin._version import get_versions
-from tableauserverclient.server.server import NAMESPACE as DEFAULT_NAMESPACE
+
+# Inlined as a string literal (rather than imported from server.server) so
+# loading `tableauserverclient` doesn't force the full server + endpoint
+# import graph to load just to read a constant. Kept in sync with the
+# NAMESPACE constant defined in tableauserverclient/server/server.py.
+DEFAULT_NAMESPACE = "http://tableau.com/api"
+
 from tableauserverclient.models import (
     BackgroundJobItem,
     CollectionItem,