[Aspire extension] Phase 8: Retire the built-in Aspire implementation

Parent epic: #9479

Depends on successful legacy migration and demonstrated extension reliability.

## Objective

Remove the maintenance-only built-in C# Aspire implementation after the extension fully owns supported Aspire workflows.

## Removal candidates

- Built-in AppHost manifest generation and parsing used only by Aspire deployment.
- `DotNetImporter` Aspire expansion behavior.
- `DotNetContainerAppTarget` and Aspire-specific deployment paths.
- Built-in generated Aspire infrastructure implementation.
- Aspire-specific child-service state and execution gating.
- Legacy errors and validation that refer to Container Apps as the Aspire host.

## Core responsibilities retained

- Lightweight bootstrap discovery for finding the Aspire extension.
- Legacy configuration recognition and routing.
- Generic extension installation and provider orchestration.
- Generic azd environments, hooks, pipelines, prompts, output, and telemetry.

## Preconditions

- New and legacy projects use the extension successfully.
- App Service and Container Apps parity is demonstrated.
- Supported polyglot scenarios are stable.
- CI/no-prompt, offline installation, environment refresh, down, and infra generation are resolved.
- Extension reliability and performance meet agreed thresholds.
- A supported rollback story exists for affected releases.

## Exit criteria

- Core contains no Aspire deployment implementation.
- Legacy project configuration routes to the extension.
- Removing built-in code does not regress the agreed baseline scenario matrix.