test(mcp): isolate MCP discovery tests from a leaked SERVER_ROOT_PATH

tests/test_litellm/proxy/test_custom_proxy.py sets SERVER_ROOT_PATH at import time (its app
mounts under a custom path) and never restores it, so in a shared shard the value leaks into the
process. The discovery routes and the 401 challenges now read SERVER_ROOT_PATH to path-insert it
where they previously ignored it, so a leaked value rewrites every resource_metadata URL and the
exact-URL assertions in the delegate, pass-through, and aggregate challenge tests fail depending
on shard order

An autouse fixture clears SERVER_ROOT_PATH for the MCP discovery tests so they deterministically
exercise the default root-mounted deployment; the tests that assert a sub-path deployment set the
value explicitly within their own body. No assertion changed; the leak was invisible before only
because the code ignored the variable

tests/test_litellm/proxy/_experimental/mcp_server/conftest.py

@@ -0,0 +1,22 @@
+import os
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _hermetic_server_root_path():
+    """Isolate MCP discovery tests from a leaked ``SERVER_ROOT_PATH``.
+
+    ``tests/test_litellm/proxy/test_custom_proxy.py`` sets ``SERVER_ROOT_PATH`` at import time
+    (its app mounts under a custom path) and never restores it, so in a shared shard the value
+    leaks into this process. The discovery routes and the 401 challenges read it, so a leaked
+    value would silently rewrite every ``resource_metadata`` URL and make these tests depend on
+    shard ordering. Clearing it here pins the default (root-mounted) deployment; a test that
+    exercises a sub-path deployment sets the value explicitly within its own body.
+    """
+    saved = os.environ.pop("SERVER_ROOT_PATH", None)
+    try:
+        yield
+    finally:
+        if saved is not None:
+            os.environ["SERVER_ROOT_PATH"] = saved