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

[vhvb1989]

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.