[Aspire extension] Phase 0: Freeze and baseline built-in Aspire support

[vhvb1989]

Parent epic: #9479

Objective

Place the existing built-in C# Aspire implementation into maintenance mode and establish the behavioral baseline that the new extension must preserve or intentionally replace.

Scope

• Document the current supported scenarios and known limitations.
• Define maintenance mode: blocking compatibility, security, and reliability fixes only.
• Identify the built-in implementation surfaces, including AppHost detection, manifest generation, importing, Bicep generation, deployment, endpoints, pipeline integration, and Aspire-specific execution gating.
• Capture representative fixtures for C# AppHosts targeting Container Apps and App Service.
• Establish baseline coverage for azd init, package, provision, deploy, up, down, infra generate, environment outputs, hooks, endpoints, and pipeline configuration.
• Record expected behavior for legacy host: containerapp plus language: dotnet projects.

Deliverables

• A documented support and maintenance policy.
• A scenario matrix identifying current behavior, limitations, and test coverage.
• Stable regression fixtures that can later run against both built-in and extension implementations.

Exit criteria

• The team can identify whether a future change is a built-in hotfix or new extension development.
• Existing behavior is sufficiently covered to detect compatibility regressions during migration.