fix(app-builder): use cross-spawn for pnpm on Windows

[falkenbergo]

Problem

On Windows, spawning pnpm with Node's native child_process.spawn(pnpm, ...) could fail with spawn pnpm ENOENT when the app-builder server runs pnpm install and pnpm exec vite ….

Solution

• Use cross-spawn for those subprocess calls so Windows resolves pnpm.cmd/shims correctly.
• Add @types/cross-spawn as a devDependency and relax devProcess typing to match cross-spawn's returned ChildProcess; use optional chaining when piping stdout/stderr.

Testing

• pnpm build succeeds in sdk/app-builder.

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Note

Medium Risk
Moderate risk because it changes how pnpm install and the Vite dev server are spawned and how their streams are handled, which could affect session setup/startup across platforms.

Overview
Fixes Windows failures when starting app-builder sessions by replacing Node’s child_process.spawn usage with cross-spawn for pnpm commands.

Updates process typing (ChildProcess) and log piping to tolerate missing stdout/stderr streams via optional chaining, and adds cross-spawn plus @types/cross-spawn to dependencies/lockfile.

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