[Aspire extension] Phase 6: Make the extension the default for new Aspire projects

[vhvb1989]

Parent epic: #9479

Depends on extension workflow and language readiness.

Objective

Make microsoft.azd.aspire the standard implementation selected by azd init for newly detected Aspire projects.

Scope

• Generate the new project contract:

  services:
    app:
      host: aspire
      project: ./apphost
  infra:
    provider: aspire

• Keep language optional.

• Add aspire to stable and alpha azure.yaml schemas and documentation.

• Implement transparent first-party provider discovery and installation.

• Define behavior for interactive, --no-prompt, offline, and restricted-network environments.

• Decide whether the extension and/or Aspire CLI must be bundled to preserve zero-setup and offline scenarios.

• Add compatible extension version requirements to generated projects where appropriate.

• Ensure provider auto-install does not conflict with private built-in fallback behavior.

• Update telemetry to distinguish public host, resolved provider, implementation, language, Aspire CLI version, and manifest-derived compute target.

Exit criteria

• New Aspire projects initialized by azd use the extension by default.
• Installation and failure behavior is deterministic in interactive and CI environments.
• Official schemas, telemetry documentation, and user documentation describe the new contract.