fix: patch user_id in header

[cpacker]

#1825 (comment)

---

letta server

When you use an invalid user_id that doesn't exist:

% curl --request GET \
  --url http://localhost:8283/v1/agents/ \
  --header 'user_id: banana'
{"detail":"User with id banana not found"}

When you don't specify a user_id (server uses the default):

% curl --request GET \
  --url http://localhost:8283/v1/agents/
[{"description":null,"metadata_":{"human":"basic","persona":"sam_pov"},"user_id":"user-00000000","id":"agent-16ffdc6b-fc6b-42...

When you specify a user_id (here I'm using the default):

% curl --request GET \
  --url http://localhost:8283/v1/agents/ \
  --header 'user_id: user-00000000'
[{"description":null,"metadata_":{"human":"basic","persona":"sam_pov"},"user_id":"user-00000000","id":"agent-16ffdc6b-fc6b-427d-aeac