distribute_deal_via_ssp: validate deal_type and cpm, and map Index 4xx responses to client errors #122

Pre-existing on main, found while reviewing #111:

• An unknown deal_type (e.g. buyer-agent's "PD") silently falls back to PMP, so the deal is created as an auction deal and imported back as PA with a bid floor.
• A negative cpm is accepted.
• An Index Exchange 4xx from create_deal surfaces as a 500 instead of a 4xx/502 with the upstream message.
  Proposal: reject unknown deal_type values with a 400, validate cpm >= 0, and map upstream 4xx to a clear error. Follow-up to fix: align the Index Exchange connector with the real /v3/deals API #111.