Suppress agent initialization for aspire-test templates

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

After creating an `aspire-test` project, `aspire new` runs the generic agent-initialization flow. Users can currently avoid it explicitly:

```console
aspire new aspire-test --suppress-agent-init
```

Requiring this option for a test-project template is unintuitive because `aspire-test` adds a test project rather than creating a new Aspire application.

### Describe the solution you'd like

Suppress agent initialization by default for all `aspire-test` variants (MSTest, NUnit, and xUnit). Users can run `aspire agent init` separately when desired. Other templates should retain their existing behavior.

Acceptance criteria:

- `aspire new aspire-test` does not prompt for or run agent initialization by default.
- All supported test frameworks behave consistently.
- Other templates remain unchanged.
- Interactive and non-interactive behavior is covered by tests.

### Additional context

This was raised by @adamint in [a review comment on PR #19668](https://github.com/microsoft/aspire/pull/19668#discussion_r3970334488): creating an integration-test project prompted for AI agents even though it is typically added to an existing Aspire app.

Start with the `aspire new aspire-test` flow and compare it with the separate `aspire agent init` command, including the review context in PR #19668. Update the behavior for MSTest, NUnit, and xUnit variants, then add interactive and non-interactive coverage showing that other templates remain unchanged.

Written by the indexing model from the issue text.