fix(actions): improve errors when ${{ needs... }} is used in strategy.matrix incorrectly (#10298)

Three fixes are presented together in this PR:
- When a `strategy.matrix` entry in an Action job contains `${{ needs.some-job.outputs.some-output }}`, if that output *never* becomes available, different error messages will be presented if `some-job` isn't found or if `some-output` isn't found.  This clarifies an error message that was previously "it could be this, or it could be this".
- In the error case described in the previous point, other jobs in the workflow could continue running or could be left "blocked" forever.  A centralized `FailRunPreExecutionError` function ensures that all incomplete jobs in the run are failed in this case.
- In a rare error case when a job referenced another job in `strategy.matrix` but no other jobs were defined in the workflow, the job would be marked as blocked forever because the `job_emitter` code would never be invoked to detect this case.  A new `consistencyCheckRun` function for a newly created `ActionRun` adds a location to perform a pre-execution check for this case so that the run can be failed.

These fixes are all interconnected around the refactor for the `FailRunPreExecutionError`, causing them to be bundled rather than individual PRs.

## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.
    - These are fixes to an unreleased feature and don't require release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/10298
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Co-committed-by: Mathieu Fenniak <mathieu@fenniak.net>

models/actions/run_job.go

@@ -43,6 +43,8 @@ type ActionRunJob struct {
 	Stopped           timeutil.TimeStamp
 	Created           timeutil.TimeStamp `xorm:"created"`
 	Updated           timeutil.TimeStamp `xorm:"updated index"`
+
+	workflowPayloadDecoded *jobparser.SingleWorkflow `xorm:"-"`
 }
 
 func init() {
@@ -256,13 +258,34 @@ func (job *ActionRunJob) StatusDiagnostics(lang translation.Locale) []template.H
 	return diagnostics
 }
 
-// Checks whether the target job is an `(incomplete matrix)` job that will be blocked until the matrix is complete, and
-// then regenerated and deleted.
-func (job *ActionRunJob) IsIncompleteMatrix() (bool, error) {
+func (job *ActionRunJob) decodeWorkflowPayload() (*jobparser.SingleWorkflow, error) {
+	if job.workflowPayloadDecoded != nil {
+		return job.workflowPayloadDecoded, nil
+	}
+
 	var jobWorkflow jobparser.SingleWorkflow
 	err := yaml.Unmarshal(job.WorkflowPayload, &jobWorkflow)
 	if err != nil {
-		return false, fmt.Errorf("failure unmarshaling WorkflowPayload to SingleWorkflow: %w", err)
+		return nil, fmt.Errorf("failure unmarshaling WorkflowPayload to SingleWorkflow: %w", err)
 	}
-	return jobWorkflow.IncompleteMatrix, nil
+
+	job.workflowPayloadDecoded = &jobWorkflow
+	return job.workflowPayloadDecoded, nil
+}
+
+// If `WorkflowPayload` is changed on an `ActionRunJob`, clear any cached decoded version of the payload.  Typically
+// only used for unit tests.
+func (job *ActionRunJob) ClearCachedWorkflowPayload() {
+	job.workflowPayloadDecoded = nil
+}
+
+// Checks whether the target job is an `(incomplete matrix)` job that will be blocked until the matrix is complete, and
+// then regenerated and deleted.  If it is incomplete, and if the information is available, the specific job and/or
+// output that causes it to be incomplete will be returned as well.
+func (job *ActionRunJob) IsIncompleteMatrix() (bool, *jobparser.IncompleteNeeds, error) {
+	jobWorkflow, err := job.decodeWorkflowPayload()
+	if err != nil {
+		return false, nil, fmt.Errorf("failure decoding workflow payload: %w", err)
+	}
+	return jobWorkflow.IncompleteMatrix, jobWorkflow.IncompleteMatrixNeeds, nil
 }