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

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.