fix(proxy): always merge caller-supplied tags into request metadata

Caller-supplied tags (`x-litellm-tags` header, body `tags`, `metadata.tags`)
were silently dropped unless the key/team had
`metadata.allow_client_tags: true` set. Restore the documented behavior:
tags from the request always flow into `metadata.tags` and union with any
admin-configured static tags from key/team/project metadata.

Removes the `allow_client_tags` opt-in flag from the pre-call pipeline.
The flag was only ever read here; it has no schema or endpoint footprint,
so leftover values in existing key metadata are inert.

Test cleanup mirrors the simplification: drop the three tests that
verified the strip-when-not-opted-in path, drop the `allow_client_tags`
fixture lines from the merge/union tests.

tests/proxy_unit_tests/test_proxy_utils.py

@@ -167,13 +167,9 @@ 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,
@@ -909,13 +905,12 @@ async def test_add_litellm_data_to_request_duplicate_tags(
     mock_request.headers = {}
     mock_request.state = State()
 
-    # Setup key with tags in metadata. Opt into client-supplied tags so the
-    # request_tags are preserved for the merge under test.
+    # Setup key with tags in metadata.
     user_api_key_dict = UserAPIKeyAuth(
         api_key="test_api_key",
         user_id="test_user_id",
         org_id="test_org_id",
-        metadata={"tags": key_tags, "allow_client_tags": True},
+        metadata={"tags": key_tags},
     )
 
     # Setup request data with tags