[Dashboard] Prune incompatible-schema run directories

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

`DashboardRunStore.LoadRuns` excludes metadata whose schema version differs from the current version. `PruneRuns` only iterates the successfully loaded `_runs` snapshot.

After a Dashboard schema upgrade, older-schema run directories become invisible to both selection and retention, so storage can grow beyond the advertised run limit.

### Expected Behavior

Incompatible historical runs should remain non-selectable but still participate in retention cleanup.

### Steps To Reproduce

1. Create `Run` history with one Dashboard schema version.
2. Start a Dashboard whose schema version has advanced.
3. Create enough new runs to trigger retention.
4. The old-schema directories are never selected as pruning candidates.

### Exceptions (if any)

N/A

### Aspire doctor output

```plain
Aspire CLI 13.6.0-pr.18924.g5ee63941
Windows 10.0.26200 and Ubuntu 24.04.3
```

### Anything else?

Related to #18924. Source: `src/Aspire.Dashboard/ServiceClient/DashboardRunStore.cs:336-369,440-465`. This was mentioned during PR review and remains on current `main`.

Start in src/Aspire.Dashboard/ServiceClient/DashboardRunStore.cs, especially LoadRuns and PruneRuns at lines 336-369 and 440-465. Trace how incompatible schema metadata is excluded from _runs, then verify that retention cleanup still considers those directories while leaving them non-selectable. Done means old-schema run directories are removed when the run limit is exceeded.

Written by the indexing model from the issue text.