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>

services/actions/workflow.go

@@ -140,11 +140,17 @@ func DispatchActionWorkflow(ctx reqctx.RequestContext, doer *user_model.User, re
 	workflow := &model.Workflow{
 		RawOn: singleWorkflow.RawOn,
 	}
+	workflowDispatch := workflow.WorkflowDispatchConfig()
+	if workflowDispatch == nil {
+		return 0, util.ErrorWrapTranslatable(
+			util.NewInvalidArgumentErrorf("workflow %q has no workflow_dispatch event trigger", workflowID),
+			"actions.workflow.has_no_workflow_dispatch", workflowID,
+		)
+	}
+
 	inputsWithDefaults := make(map[string]any)
-	if workflowDispatch := workflow.WorkflowDispatchConfig(); workflowDispatch != nil {
-		if err = processInputs(workflowDispatch, inputsWithDefaults); err != nil {
-			return 0, err
-		}
+	if err = processInputs(workflowDispatch, inputsWithDefaults); err != nil {
+		return 0, err
 	}
 
 	// ctx.Req.PostForm -> WorkflowDispatchPayload.Inputs -> ActionRun.EventPayload -> runner: ghc.Event