[Impl] No field identifies the advertiser or brand a commitment is for #126

What did you build?

An implementation that parses and validates OpenProposal proposals, line items and commitments end to end.

What you tried to express, and what happened

I tried to resolve which party a buyer's commitment binds. The only buyer-side identifiers in the draft are brief_ref, eligible_bidders[], and the supplied half of buyer_requirements (dsp_seat_id, dsp_id, seat_owner_entity). seat_owner_entity names who owns the DSP seat, which is often the agency rather than the advertiser. Nothing says which advertiser or brand a commitment binds. A commitment therefore carries an amount and dates with no named party it's for.

What you did instead

Took the advertiser and brand from outside the object, from the authenticated context of the request, and refused to proceed when they're missing.

Suggested resolution: either add an advertiser/brand identity (with its namespace) to the proposal or the commitment, or state that party identity belongs to the transport binding and is out of scope for the object. #57's authorization-evidence point is adjacent but separate: that is about who approved the spend, this is about who the spend is for.

Fields involved

commitment, buyer_requirements, brief_ref, eligible_bidders[]

How blocking was it?

Workaround required, but it works

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