[Dashboard] Reject numeric persistence mode values

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

`PostConfigureDashboardOptions` uses `Enum.TryParse` followed by `Enum.IsDefined`, so numeric strings that map to defined enum members are accepted.

For example, `--persistence 1` initializes the Dashboard in `Run` mode even though user-facing validation lists only `None`, `Run`, and `Resume`.

### Expected Behavior

Only the documented named values should be accepted.

### Steps To Reproduce

1. Run `aspire dashboard run --persistence 1`.
2. Inspect the Dashboard startup log.
3. The Dashboard initializes in `Run` mode instead of rejecting the value.

### 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/Configuration/PostConfigureDashboardOptions.cs:39-50`. The same behavior remains on current `main`.

Start in src/Aspire.Dashboard/Configuration/PostConfigureDashboardOptions.cs, especially lines 39-50, and reproduce the behavior with `aspire dashboard run --persistence 1`. Done means numeric strings are rejected while the documented named values None, Run, and Resume remain accepted.

Written by the indexing model from the issue text.