Meta ads: whitelist promotedObject.customEventStr on POST /v1/ads/create (custom_event_str passthrough)

We need to optimize Meta conversion campaigns on pixel custom events (e.g. lead_ok) via the API. Meta supports this natively as:

promoted_object: { pixel_id: "...", custom_event_type: "OTHER", custom_event_str: "lead_ok" }

Your campaign READ endpoints already return custom_event_str on ad sets — our ad account (act_640784214657998) has many live campaigns using OTHER + custom_event_str=lead_ok, created in Ads Manager.

But POST /v1/ads/create rejects promotedObject.customEventStr with: 400 invalid_field_value — Unrecognized key: "customEventStr" (additionalProperties: false on the promotedObject schema)

Request — either one unblocks us:

1. Whitelist promotedObject.customEventStr on POST /v1/ads/create (and PUT /v1/ads/ad-sets/{id}) and forward it to Meta when customEventType=OTHER. You already return this field on reads, so the Meta-side mapping exists.

2. Even better: proxy GET/POST act_{id}/customconversions, or add ensureCustomConversion({ adAccountId, pixelId, eventName }) using your Marketing Partner token. This fits your current customConversionId model and removes the need for our users to hold an ads_management Graph token just to wrap their own pixel events.

Context: we send these events server-side via CAPI (sendConversions works great), they're confirmed in Events Manager, and the only missing piece is selecting them as the optimization event at campaign creation — exactly like Ads Manager's "custom conversion / custom event" dropdown.