Conformance coverage for match keys before graduating media_buy.match_keys #8451

Follow-up to #8446. These are required before media_buy.match_keys leaves experimental status.

#8446 adds these rules in the docs only, because none of them can be checked within one response schema:

• Every Product.identity.match_keys entry is covered by a seller-wide media_buy.supported_match_keys entry with the same uid_type and vendor, whose scopes and uses include it.
• A uid_type with audience uses appears in audience_targeting.supported_uid_types; one with conversion_attribution appears in conversion_tracking.supported_uid_types, when those lists are present.
• Every scope of a frequency_cap key appears in frequency_capping.supported_per_units, when that list is present.
• A product identity_match key's uid_type appears in an Identity Match provider's uid_types on that product.
• A product that declares match_keys comes from a seller listing media_buy.match_keys and media_buy.product_identity.

Scope: training-agent fixtures that declare match keys, plus a storyboard step that checks these rules across get_adcp_capabilities and get_products.