get-creative-features: request missing account context for billing

[bokelley]

Problem

creative/get-creative-features-request.json has no account field:

{
  "properties": {
    "creative_manifest": { "$ref": "../core/creative-manifest.json" },
    "feature_ids": { "type": "array" },
    "context": { "$ref": "../core/context.json" },
    "ext": { "$ref": "../core/ext.json" }
  },
  "required": ["creative_manifest"]
}

Creative governance agents that charge per-evaluation (e.g., brand safety vendors, creative compliance services) have no mechanism to:

1. Know which account to bill for the evaluation
2. Apply per-account rate cards or access controls
3. Correlate the evaluation with subsequent report_usage calls

Compare get-signals-request.json which has account_id for exactly this purpose: "When provided, the signals agent returns per-account pricing options if configured."

Proposed Fix

Add optional account: $ref: account-ref.json to get-creative-features-request.json:

"account": {
  "$ref": "../core/account-ref.json",
  "description": "Account context for this evaluation. When provided, the governance agent may apply per-account pricing or access controls."
}

This is consistent with the broader AdCP pattern of passing account context to vendor agents for billing correlation.

Affected Schemas

• creative/get-creative-features-request.json

[bokelley]

Resolved in #1252.