feat(actions): add job summaries (GITHUB_STEP_SUMMARY) (#37500)

- Add GitHub-style Actions **job summaries** support
(`GITHUB_STEP_SUMMARY` / `workflow/SUMMARY.md`) and render them on the
run Summary view.
- Store uploaded summaries internally in the DB (not as downloadable
artifacts).
- Add runtime-token endpoint for runners to upload summaries:
- `PUT
/api/actions_pipeline/_apis/pipelines/workflows/{run_id}/jobs/{job_id}/summary`
- Advertise support to runners via `RunnerService.Declare` response
header:
  - `X-Gitea-Actions-Capabilities: job-summary`
- Devtest: extend `/devtest/repo-action-view/...` to include mock
`jobSummaries` for previewing UI rendering.

## Compatibility
- New Gitea + old runner: no summary upload → UI shows nothing (no
behavior change)
- New runner + old Gitea: capability not advertised → runner skips
upload (no behavior change)

## Screenshot:

<img width="2017" height="729"
src="https://github.com/user-attachments/assets/31f8b945-50c4-40e1-9f40-382901a53013"
/>


Fixes #23721
PR on gitea-runner https://gitea.com/gitea/runner/pulls/917

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

routers/api/actions/runner/runner.go

@@ -161,7 +161,7 @@ func (s *Service) Declare(
 		return nil, status.Errorf(codes.Internal, "update runner: %v", err)
 	}
 
-	return connect.NewResponse(&runnerv1.DeclareResponse{
+	resp := connect.NewResponse(&runnerv1.DeclareResponse{
 		Runner: &runnerv1.Runner{
 			Id:      runner.ID,
 			Uuid:    runner.UUID,
@@ -170,7 +170,11 @@ func (s *Service) Declare(
 			Version: runner.Version,
 			Labels:  runner.AgentLabels,
 		},
-	}), nil
+	})
+	// Capabilities are communicated via headers to avoid a hard dependency on a proto bump.
+	// Older runners ignore unknown headers; newer runners can use this for feature negotiation.
+	resp.Header().Set("X-Gitea-Actions-Capabilities", actions_model.RunnerCapabilities())
+	return resp, nil
 }
 
 // FetchTask assigns a task to the runner