fix(mcp): return 409 on BYOK/user-fields conflict; coerce Pydantic user_fields

- store_mcp_user_credential now catches the ValueError raised by
  store_user_credential when the (user, server) row already holds a
  user-fields payload, returning HTTP 409 instead of leaking a 500.
- coerce_user_fields now also accepts MCPUserField Pydantic instances
  (the shape used by LiteLLM_MCPServerTable.user_fields), so the
  management-layer annotation and enforcement paths no longer return
  silently empty when callers pass a parsed server record.

litellm/proxy/management_endpoints/mcp_management_endpoints.py

@@ -1918,9 +1918,19 @@ if MCP_AVAILABLE:
                 detail={"error": "User ID not found in token"},
             )
         if payload.save:
-            await store_user_credential(
-                prisma_client, user_id, server_id, payload.credential
-            )
+            try:
+                await store_user_credential(
+                    prisma_client, user_id, server_id, payload.credential
+                )
+            except ValueError as e:
+                raise HTTPException(
+                    status_code=status.HTTP_409_CONFLICT,
+                    detail={
+                        "error": "credential_conflict",
+                        "message": str(e),
+                        "server_id": server_id,
+                    },
+                )
             from litellm.proxy._experimental.mcp_server.server import (
                 _invalidate_byok_cred_cache,
             )

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

@@ -179,6 +179,28 @@ def test_coerce_user_fields_empty_when_missing():
     assert server_has_user_fields(srv) is False
 
 
+def test_coerce_user_fields_accepts_litellm_mcp_server_table():
+    """LiteLLM_MCPServerTable.user_fields is List[MCPUserField] (Pydantic
+    instances), not List[dict]. The helper must normalise both shapes so
+    the management-layer annotation / enforcement paths don't silently
+    return empty results.
+    """
+    table = LiteLLM_MCPServerTable(
+        server_id="s3",
+        transport=MCPTransport.http,
+        user_fields=[
+            {"field_key": "TOKEN", "header_name": "Authorization", "required": True},
+            {"field_key": "WS", "header_name": "X-Workspace", "required": False},
+        ],
+    )
+    assert isinstance(table.user_fields[0], MCPUserField)
+    coerced = coerce_user_fields(table)
+    assert [f["field_key"] for f in coerced] == ["TOKEN", "WS"]
+    assert server_has_user_fields(table) is True
+    missing = compute_missing_user_fields(table, None)
+    assert [f["field_key"] for f in missing] == ["TOKEN"]
+
+
 def test_compute_missing_required_only():
     srv = _gmail_server()
     missing = compute_missing_user_fields(srv, None)