test: update tag-merge tests for default-deny client tag policy

Two pre-existing tests codified the pre-fix behavior where any caller-
supplied metadata.tags would flow through to spend logs and routing:

- test_add_key_or_team_level_spend_logs_metadata_to_request exercised
  the request/key/team tag merge. Set allow_client_tags=True on the key
  metadata so the merge path is still tested under the new regime.

- test_create_file_with_nested_litellm_metadata asserted that
  litellm_metadata[tags] form-data propagated to the handler. Drop the
  tag field; the test still proves nested form-parser correctness via
  spend_logs_metadata and environment.

tests/proxy_unit_tests/test_proxy_utils.py

@@ -162,9 +162,13 @@ async def test_add_key_or_team_level_spend_logs_metadata_to_request(
 
     print(f"team_sl_metadata: {team_sl_metadata}")
     mock_request.url.path = "/chat/completions"
+    # Opt the key into client-supplied tags so request_tags are preserved
+    # and merged with admin-configured key/team tags. Without this flag,
+    # request_tags would be stripped by add_litellm_data_to_request.
     key_metadata = {
         "tags": key_tags,
         "spend_logs_metadata": key_sl_metadata,
+        "allow_client_tags": True,
     }
     team_metadata = {
         "tags": team_tags,