Read/update use a per-resource struct that embeds the model by value and adds a
readOnly max_permission field (labelReadBody, projectViewReadBody); Go and Huma
promote the embedded fields, so the body stays flat with no custom marshaler and
nothing on the shared models. The handler passes the model's Updated and the
permission to conditionalReadResponse, which folds the permission into the ETag.
Adds a webtest asserting two callers with different permission on the same label
get different ETags, plus max_permission presence assertions.
e22e169fb9