ci: trigger the Maven build on merge_group

[OliverWolffGIP]

A merge queue does not build the PR ref. It builds a candidate on refs/heads/gh-readonly-queue/main/... and dispatches the merge_group event.

Without this trigger, once a queue is enabled:

1. queue entry created → no workflow runs
2. required build / conclusion never reports
3. entry ejected at the 60-minute timeout

Net effect: nothing can ever merge into main. So the trigger has to land before the queue is enabled, not with it.

This PR is inert on its own — no queue exists on this repo yet, and merge_group events are only dispatched by one. The queue follows from cuioss-organization's merge_queue_repos.

Verified on cui-http, which has had the trigger and a live queue since 2026-08-26 — five merge_group runs on gh-readonly-queue/main/pr-* branches, all success. The reusable workflow already handles the event; no change is needed there.

Validated locally:

triggers: ['merge_group', 'push', 'pull_request', 'workflow_dispatch']

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