[Aspire extension] Phase 3: Implement microsoft.azd.aspire providers

Parent epic: #9479

Depends on the Aspire CLI contract spike and extension-driven detection support.

## Objective

Create the first-party `microsoft.azd.aspire` extension and its aggregate Aspire application providers.

## Extension capabilities

- Application detection/initialization provider.
- `aspire` service-target provider.
- `aspire` provisioning provider.
- Framework provider only where it adds value; `language` must remain optional.

## Scope

- Discover and validate AppHosts using the Aspire CLI.
- Validate and report Aspire CLI installation/version requirements.
- Model the complete Aspire application as one aggregate azd service.
- Run Aspire publish/deployment operations and stream progress through azd.
- Translate Aspire failures into structured extension errors.
- Return service, deployment, resource, dashboard, and endpoint artifacts.
- Integrate azd environment values, AppHost parameters, and cancellation.
- Register providers consistently with `extension.yaml` and extension registry metadata.
- Add cross-platform packaging and signed first-party release support.

## Non-goals

- Reproduce the built-in importer's synthetic child services.
- Reimplement Aspire's internal language or AppHost protocols.
- Remove the existing built-in implementation.

## Exit criteria

- The extension can detect and run an end-to-end C# Aspire scenario through azd.
- Provider registration, lifecycle, errors, artifacts, and cancellation behave like other first-party extensions.