Spec: Add property_targeting_allowed flag to Product

[bokelley]

Summary

When implementing property list filtering in get_products, we discovered that Product lacks a flag to indicate whether property-level targeting is allowed. This distinction is necessary for correct filtering behavior.

Problem

Some products are "all or nothing" (run of network) - the buyer must accept all properties. Other products allow the buyer to filter down to a subset of properties.

Without a flag, the sales agent cannot determine which products can be filtered.

Example

Product 1: properties [a, b, c], property_targeting_allowed=false
Product 2: properties [a, b, c], property_targeting_allowed=true  
Product 3: properties [d, e, f], property_targeting_allowed=true

Buyer calls: get_products(property_list=reference_to_[a, b, g])

Expected filtering:
- Product 1: EXCLUDED (has c, but can't drop it - all or nothing)
- Product 2: INCLUDED (intersection [a, b] is non-empty, can subset)
- Product 3: EXCLUDED (no intersection with [a, b, g])

Result: Only Product 2 returned

Proposed Solution

Add a field to Product:

class Product:
    # ... existing fields ...
    
    property_targeting_allowed: bool = Field(
        default=True,
        description="Whether buyers can filter this product to a subset of its publisher_properties. "
                    "When false, the product is 'all or nothing' - buyers must accept all properties."
    )

Alternative Approaches

1. Infer from selection_type - If publisher_properties uses selection_type: "all", maybe that implies no sub-targeting? But this conflates two different concepts.

2. Separate product types - Have different product categories, but this seems overly complex.

3. Capabilities flag - Add to get_adcp_capabilities response, but this is product-level, not agent-level.

Context

Discovered while implementing property list filtering per AdCP v3 spec in the reference sales agent implementation (prebid/salesagent).

The PropertyListReference in GetProductsRequest allows buyers to pass a list of allowed properties, but without knowing which products support sub-targeting, the filtering logic is ambiguous.

[bokelley]

Closing - created in wrong repo. Moving to adcontextprotocol/adcp (the spec repo).