fix: fail init when package manager is unavailable - #2849

Summary

init --pm <name> currently logs an error and returns normally when the requested package manager is unavailable. That makes the CLI report a successful exit status even though no project was created.

Throw a CLIError instead so the existing command error handler reports the failure and exits nonzero. This changes only the unavailable explicit-package-manager path.

Test Plan

• Added a regression test that requests an unavailable Yarn installation.
• Verified the regression fails on unmodified main: the initialization promise resolves to undefined instead of rejecting.
• Verified the focused regression passes with this change.
• yarn test --selectProjects unit --runInBand: 57 suites passed, 315 tests passed, 1 todo, 62 snapshots passed.
• yarn lint: passed with the two pre-existing warnings in openURLMiddleware.test.ts and commands/init/version.ts.
• yarn build: passed for all packages.
• git diff --check: passed.
• The broad unfiltered Jest command also selects the repository's end-to-end project; it was stopped after its existing init TestProject --install-pods process waited indefinitely. No broad end-to-end result is claimed.

Checklist

• Documentation is up to date (no documentation change is needed for correcting the exit status).
• Follows commit message convention described in CONTRIBUTING.md.
• For functional changes, my test plan has linked these CLI changes into a local react-native checkout (instructions).