[VS Code] Preselect the CLI-default Aspire skills and agent assets during project creation #19629

The VS Code extension should show the same default selections as the CLI when aspire new or aspire init reaches the skill-location and skill Quick Picks. Users can still deselect them, but the default experience should be opt-out instead of opt-in.

Investigation

The CLI already owns and computes the correct defaults in AgentInitCommand; we do not need a second defaults API or a copied list in the extension.

Today AgentInitCommand passes those choices through IInteractionService.PromptForSelectionsAsync(..., preSelected: ...):

• .agents/skills is the default skill location.
• Bundle-sourced Aspire skills that apply to the detected AppHost language are selected by default.
• aspire new excludes the one-time aspireify setup skill because the template already created the AppHost.
• aspire init includes aspireify because wiring the existing applications is the natural follow-up.
• CLI-defined skills such as playwright-cli and dotnet-inspect remain opt-in.
• MCP configuration remains intentionally opt-in.

The values are lost only on the VS Code path: ExtensionInteractionService.PromptForSelectionsAsync explicitly drops preSelected before calling IExtensionBackchannel.PromptForSelectionsAsync. The extension endpoint then receives only string[] choices and calls showQuickPick with no picked items.

Implementation

1. Extend IExtensionBackchannel.PromptForSelectionsAsync and ExtensionBackchannel.PromptForSelectionsAsync to accept the CLI-computed preSelected values, format them with the same formatter as the choices, and send them as an optional trailing promptForSelections RPC argument.
2. Forward preSelected from ExtensionInteractionService.PromptForSelectionsAsync instead of dropping it.
3. Update extension/src/server/interactionService.ts so promptForSelections accepts an optional preselected-label array, creates QuickPickItem values with picked: true where appropriate, and maps the selected items back to their labels.
4. Keep the RPC addition backwards compatible: an older CLI omits the optional argument, while an older extension ignores the extra trailing JavaScript argument. No capability gate should be necessary.

Tests

• Add CLI coverage in ExtensionInteractionServiceTests/TestExtensionBackchannel proving formatted preselected values cross the backchannel.
• Add extension interaction-service coverage proving default items are picked, non-default items are not, deselection is honored, and calls from an older CLI without the new argument still work.
• Add focused VS Code E2E coverage for the aspire new flow if the E2E Quick Pick helper can reliably inspect selected state; the existing fixture creation suppresses agent init, so this needs a purpose-built interaction flow rather than the current setup fixture.

Acceptance criteria

• VS Code presents the same defaults as the equivalent terminal CLI flow for both aspire new and aspire init.
• Users can deselect any default or select an opt-in item.
• The extension contains no hardcoded skill names or default list.
• Older CLI versions continue to work with the updated extension.

cc Ella Hathaway (@ellahathaway)