[VS Code] Complete setup and project creation workflows

## Goal

Make Aspire setup and project creation in VS Code reliable and discoverable across local, remote, and enterprise environments.

This is an area tracker under #19405.

## Exit criteria

- The extension resolves Aspire CLI installations when the extension host and terminal have different environments.
- `aspire doctor` identifies an unsupported or outdated Aspire extension and gives concrete remediation.
- CLI and editor-driven template and package operations can use a durable default NuGet source.
- New-project creation validates destination and folder-name conflicts before generation begins.
- The Aspire pane exposes the existing new-app and initialize-existing-codebase workflows.

## Tracked work

- #17417
- #18694
- #19340
- #19283
- #19499

## Scope

Reuse the Aspire CLI and existing extension commands rather than adding separate editor-only setup behavior. Keep advanced configuration available through the CLI and configuration files while making the common path discoverable from VS Code.