[Dashboard] Ignore live connection events while viewing historical runs

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

`SelectedDashboardClient.ConnectionStateChanged` decides whether to attach to the live Dashboard client only when a handler is registered. Long-lived layout handlers registered while viewing the live run stay attached after selecting a historical run.

A later AppHost disconnect can therefore display live reconnection UI over read-only historical data.

### Expected Behavior

Historical run views should remain independent of live AppHost connection-state changes.

### Steps To Reproduce

1. Open the Dashboard on the live run so layout connection handlers are registered.
2. Select a historical run.
3. Disconnect the live AppHost resource service.
4. Live reconnection state can be presented over the historical view.

### Exceptions (if any)

N/A

### Aspire doctor output

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

### Anything else?

Related to #18924. Source: `src/Aspire.Dashboard/ServiceClient/SelectedDashboardClient.cs:18-30`. Previously raised during PR review and still present on current `main`.

Start in src/Aspire.Dashboard/ServiceClient/SelectedDashboardClient.cs:18-30 and review the related discussion in #18924. Reproduce the sequence of opening the live run, selecting a historical run, and disconnecting the live AppHost resource service. Done means live connection-state changes no longer affect the historical view.

Written by the indexing model from the issue text.