external_id in supported_identifier_types is ambiguous with AudienceMember.external_id

[bokelley]

Problem

GetAdCPCapabilitiesResponse.audience_targeting.supported_identifier_types now includes 'external_id' as a value:

```typescript
supported_identifier_types: ('hashed_email' | 'hashed_phone' | 'external_id')[];
```

AudienceMember also has a top-level external_id: string field, described as "Buyer-assigned stable identifier for this audience member (e.g. CRM record ID, loyalty ID). Used for deduplication, removal, and cross-referencing with buyer systems."

These are two different concepts sharing a name:

1. AudienceMember.external_id — a buyer-internal deduplication key, present on every audience member, not used for ad platform matching.
2. supported_identifier_types: ['external_id'] — signals that the seller can match users using the buyer's CRM/loyalty ID directly (a closed-ecosystem capability like Amazon, Walmart, Instacart).

An agent implementation could reasonably misread supported_identifier_types: ['external_id'] as "use the AudienceMember.external_id deduplication field for matching" when the seller actually means something specific to their identity graph.

Proposed fix

Rename the supported_identifier_types value to something unambiguous:

```typescript
supported_identifier_types: ('hashed_email' | 'hashed_phone' | 'retailer_customer_id' | 'platform_customer_id')[];
```

Or keep external_id but add a JSDoc note that explicitly distinguishes it from AudienceMember.external_id:

'external_id': The seller accepts the buyer's CRM/loyalty ID as a matchable identifier for their identity graph. This is distinct from AudienceMember.external_id, which is a buyer-side deduplication key and is always required. Only declare this when the seller operates a closed ecosystem with a shared ID namespace (e.g., a retailer matching against their loyalty program).

Context

This ambiguity was introduced when external_id was promoted from a UIDType union member to a top-level required field on AudienceMember in 2.5.x.

[bokelley]

Resolved in PR #1251. Removed platform_customer_id from supported_identifier_types enum and added supports_platform_customer_id boolean to audience targeting capabilities.