fix(actions): show run index in run view and fix summary graph height (#38165)

- Display the per-repository run number as `#N` next to the run title in
the run view, matching the runs list and GitHub
- Add the run `Index` to the run view API response (and the devtest
mock) to support that
- Restore the summary panel's `flex: 1` so the workflow graph fills the
right-column height even when a run has no job summaries
- Keep the job-summary section content-sized so it doesn't compete with
the graph for height
- Gate the devtest mock job summaries to a subset of runs so the devtest
page also exercises the no-summary layout

<img width="521" height="232" alt="image"
src="https://github.com/user-attachments/assets/a1f2f20b-65bd-4d98-ba6a-b8135580a6de"
/>

web_src/js/components/ActionRunView.ts

@@ -107,6 +107,7 @@ export function buildJobsByParentJobID(jobs: ActionsJob[]): Map<number, ActionsJ
 export function createEmptyActionsRun(): ActionsRun {
   return {
     repoId: 0,
+    index: 0,
     link: '',
     viewLink: '',
     title: '',