fix(actions): fail unexpandable reusable workflow callers and decouple the job emitter's cross-run processing (#38565)

## Changes

### 1. Handle reusable workflow expansion failures

If a reusable workflow caller job is invalid (e.g. uses a workflow with
syntax error), the job emitter should mark it as failed instead of
retrying.

Related:
https://github.com/go-gitea/gitea/pull/38518#discussion_r3608882095

### 2. No longer process concurrent run inline

**Before**: If a run(R1)'s status change unlocks another blocked run(R2)
via concurrency group, the job emitter will process R2 in R1's
transaction.

**Current**: No longer process R2 inline and emit the ID of R2 to let
another pass process it.

models/actions/run_job.go

@@ -333,6 +333,14 @@ func GetDirectChildJobsByParent(ctx context.Context, parentJob *ActionRunJob) (A
 	return jobs, nil
 }
 
+// DeleteDirectChildJobsByParent deletes the direct child jobs of a parent job.
+func DeleteDirectChildJobsByParent(ctx context.Context, parentJob *ActionRunJob) error {
+	_, err := db.GetEngine(ctx).
+		Where("run_id=? AND parent_job_id=?", parentJob.RunID, parentJob.ID).
+		Delete(new(ActionRunJob))
+	return err
+}
+
 // CollectAllDescendantJobs returns every job in `allJobs` that lives under parent's subtree (recursively), excluding `parent` itself
 func CollectAllDescendantJobs(parent *ActionRunJob, allJobs []*ActionRunJob) []*ActionRunJob {
 	parents := map[int64]bool{parent.ID: true}