### Description

`POST /team/member_update` accepts `max_budget_in_team: null` and clears the budget link on the underlying `LiteLLM_TeamMembership` row (both `budget_id` and the joined `litellm_budget_table` become null). The update returns HTTP 200 and `/team/info` reflects the cleared state. The DB schema permits this — `litellm_budget_table` is a nullable FK.

The `LiteLLM_TeamMembership` Pydantic model in v1.85+, however, treats `litellm_budget_table` as **required** (not `Optional`). Any subsequent request from a member of the affected team fails with:

``` 1 validation error for LiteLLM_TeamMembership litellm_budget_table Field required [type=missing, input_value="REDACTED_BY_LITELLM", input_type=dict] For further information visit https://errors.pydantic.dev/2.12/v/missing ```

The error happens in the auth/load path on every request, so **every virtual key belonging to a member of the affected team is bricked** until the member budget is set back to a non-null value. This includes existing keys created before the membership was cleared.

### Repro

1. Create a team with a pool budget:

```bash curl -X POST $LITELLM/team/new \ -H "Authorization: Bearer $MASTER_KEY" …