Graduate list handling events endpoint to stable 2.15

[faizzed]

What

Graduates the Conversations → list handling events endpoint (GET /conversations/{id}/handling_events, listHandlingEvents) from Preview to stable 2.15 in the source-of-truth spec.

Adds to descriptions/2.15/api.intercom.io.yaml:

• the endpoint path
• the handling_event and handling_event_list response schemas
• the teammate_reference schema (dependency of the response — previously only in descriptions/0/)

Endpoint is kept in descriptions/0/ (Unstable) too — it remains served on Intercom-Version: Preview, so existing Preview integrations are unaffected.

Additions-only (124 insertions, 0 deletions); spec parses as valid YAML; all $refs resolve within the 2.15 spec (error and intercom_version already present).

Postman collections regenerate automatically on merge to main (generate_postman_collection.yml). SDKs are unaffected (Fern stable source is 2.14; the endpoint is already in the unstable descriptions/0/ SDK source).

Why

Per #team-integration-capabilities thread, the API team confirmed the endpoint can go straight into the current stable version (2.15). Requesting customer: Preply.

Companion PRs (three-repo set)

1. intercom/intercom#526927 — monolith: moves AddHandlingEventsApi into the 2.15 version block
2. this PR — Intercom-OpenAPI source spec (feeds SDKs + Postman)
3. intercom/developer-docs#987 — published reference + changelog

Land this PR first/alongside #987 so the published docs aren't hand-edited ahead of their source.

[junaid-zaidi-intercom]

LGTM!