Fix legacy user context menu loading

[AboMeezO]

Fixes legacy user context menu commands in @commandkit/legacy.

The loader already marks ApplicationCommandType.User commands as userContextMenu, but the plugin wrapper was checking command.messageContextMenu before wiring the user handler. As a result, user context menu commands could load/register without a runnable handler.

Changes:

• Wire legacy user context menu handlers from command.userContextMenu.
• Add regression coverage for legacy message/user context menu loading, registration data, and user handler execution.
• Add a small test setup for the legacy package and keep test files out of build output.

Checked with:

• pnpm --filter @commandkit/legacy test
• pnpm --filter @commandkit/legacy check-types
• pnpm --filter @commandkit/legacy build
• pnpm test src/app/register/CommandRegistrar.context-menu.test.ts src/app/handlers/AppCommandHandler.hierarchical.test.ts from packages/commandkit

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