fix(actions): reject workflow_dispatch for workflows without that trigger (#37660)

## Summary

Fixes #37528

This PR makes the workflow dispatch API reject workflows that do not
declare `workflow_dispatch`. Previously, `POST
/repos/{owner}/{repo}/actions/workflows/{workflow_id}/dispatches` could
create an `ActionRun` for a workflow that only declared another event
such as `push`.

The service now validates that the target workflow has a
`workflow_dispatch` trigger before inserting the run. The API maps that
validation failure to `422 Unprocessable Entity`, matching existing
validation failures in this handler.

The regression test creates a push-only workflow, dispatches it through
the public API, asserts the `workflow_dispatch` validation message, and
verifies that no run was inserted.

## Disclosure

Developed with assistance from OpenAI Codex.

---------

Co-authored-by: Nicolas <bircni@icloud.com>