[Aspire extension] Phase 6: Make the extension the default for new Aspire projects

Parent epic: #9479

Depends on extension workflow and language readiness.

## Objective

Make `microsoft.azd.aspire` the standard implementation selected by `azd init` for newly detected Aspire projects.

## Scope

- Generate the new project contract:

```yaml
services:
app:
host: aspire
project: ./apphost
infra:
provider: aspire
```

- Keep `language` optional.
- Add `aspire` to stable and alpha `azure.yaml` schemas and documentation.
- Implement transparent first-party provider discovery and installation.
- Define behavior for interactive, `--no-prompt`, offline, and restricted-network environments.
- Decide whether the extension and/or Aspire CLI must be bundled to preserve zero-setup and offline scenarios.
- Add compatible extension version requirements to generated projects where appropriate.
- Ensure provider auto-install does not conflict with private built-in fallback behavior.
- Update telemetry to distinguish public host, resolved provider, implementation, language, Aspire CLI version, and manifest-derived compute target.

## Exit criteria

- New Aspire projects initialized by azd use the extension by default.
- Installation and failure behavior is deterministic in interactive and CI environments.
- Official schemas, telemetry documentation, and user documentation describe the new contract.