Wholesale product create should validate forecast shape used by create_media_buy

[bokelley]

Problem

A wholesale product can be created successfully with a legacy/simple forecast shape, but later create_media_buy fails when salesagent rehydrates the product into the stricter AdCP Product model.

Observed in local managed-PSA e2e:

1. Tenant-management wholesale-products:validate accepts a draft with:

{
  "forecast": { "impressions": 100000 }
}

2. Tenant-management wholesale-products creates the product.
3. Buyer-protocol create_media_buy for that product fails with:

3 validation errors for Product
forecast.points: Field required
forecast.method: Field required
forecast.currency: Field required

Expected

The write path should either:

• reject invalid/legacy forecast shapes during validate/create, with an actionable error, or
• normalize the simple legacy shape into the current structured forecast shape before persisting.

Why it matters

Agentic/Murph uses tenant-management wholesale product APIs as the write path. A product that validates and creates successfully should not become unusable only when a buyer later calls create_media_buy.

[bokelley]

Correction from Agentic e2e follow-up: Agentic should not be caller-authoring forecast in wholesale product setup. The write path should describe what the inventory bundle supports; forecast should be system/ad-server/reporting-derived read-side metadata or managed through a separate explicit operator override. We confirmed salesagent accepts an inventory-only wholesale product payload. The bug to preserve here is narrower: optional/system-owned forecast metadata should not become a hidden create_media_buy prerequisite, and the authoring API/spec should make the ownership boundary clear.