fix: Use repo package manager for generate (#13089)
Why
`turbo generate` always shells through `npx`, which makes npm enforce
`devEngines.packageManager` even in pnpm repos. That breaks generation
for repos that require pnpm via `devEngines`.
Closes #13087.
What
Detect the repo package manager before launching `@turbo/gen` and use
that manager's one-off package execution command. npm and Yarn v1 keep
the existing `npx` behavior.
How
Added targeted coverage for pnpm command selection and a regression case
with `devEngines.packageManager` plus `pnpm-lock.yaml` but no top-level
`packageManager` field.
Verified with `cargo test -p turborepo-lib commands::generate`, `cargo
fmt --check`, and pre-push hooks.
