Bulk Update Consumption Limits

Bulk platform-token limit update.

Both limit fields are tri-state, keyed off presence rather than value:

• omitted -> leave that limit unchanged
• explicit null -> remove that limit
• number -> set that limit

model_dump(exclude_unset=True) is the only thing distinguishing the first two cases, so neither limit field may carry a non-None default, and neither may use Field(gt=0) — a pydantic rejection would 422 with a list detail, while the sibling bulk endpoints 400 with a string, which is the shape the CAMS UI's error handling expects. Range checks are therefore done manually below.