Consider prompting file-based AppHosts to opt into the CLI bundle during aspire update

## Summary

Consider having `aspire update` detect file-based C# AppHosts that are not using the CLI bundle and prompt the user to opt in by adding:

```csharp
#:property AspireUseCliBundle=true
```

File-based AppHost templates currently enable the bundle, but existing apps can still encounter the ASPIRE010 warning without an obvious automated migration path.

## First step

Before implementing a prompt, enumerate the possible project states and assess whether they can be distinguished reliably enough for a safe migration.

Scenarios to consider include:

- An older file-based AppHost that has never made a bundle choice.
- An AppHost that already explicitly enables the bundle.
- A user who intentionally opted out of the bundle. They should not be prompted again every time they run `aspire update`.
- Repeated updates after the user accepts or declines the prompt.
- Non-interactive update flows.
- AppHost files whose directives or formatting make automated edits ambiguous.

The initial outcome can be a feasibility/design assessment before committing to an implementation.

Related diagnostic: https://aspire.dev/diagnostics/aspire010/

_Created from a [Microsoft Teams conversation](https://teams.microsoft.com/l/message/19%3AuuDJgMCR7JeFSM-0RK2xRW5mAq2WFNfUK0TLmYDvAU01%40thread.tacv2/1788288965460?tenantId=72f988bf-86f1-41af-91ab-2d7cd011db47&groupId=f2e0b0b6-f130-46fa-8ec6-2ef4a7168ed2&parentMessageId=1788288965460)._

Start with the aspire update command and the ASPIRE010 diagnostic, then enumerate how file-based C# AppHosts represent bundle choices. Assess whether opt-in, opt-out, repeated updates, non-interactive flows, and ambiguous directives can be distinguished safely; done means a feasibility or design assessment covering these scenarios.

Written by the indexing model from the issue text.