Upserts an assessment keyed on (provider_type, external_id) for the candidate-position.
When authenticated with a partner API key, provider_type is set from the partner registry
and provider_name in the body is ignored. Customer tokens may set provider_name when no
partner key is present.

All result content is carried in blocks, and the array's order is the order Breezy renders
it in. Every block type may appear any number of times, and blocks may be empty or omitted
while results are pending.

A write replaces the entire blocks array. Blocks carry no identity, so nothing is
merged: each write must state the complete result body as of that moment. A request that
omits blocks clears any blocks stored previously.

Blocks are structured values only. Breezy owns every presentation decision — bar
proportions, ring geometry, severity icon and color — so no markup, styling or imagery can
be supplied through this endpoint.