fix: add timeout to prevent bootstrapping from hanging indefinitely

[Statyk7]

Summary

• Adds a 120-second timeout to the Mason hook's preGen call during bootstrapping, which internally runs dart pub get for hook dependencies
• Adds a 120-second timeout to _Cmd.run() (used by Dart.pubGet() and Flutter.pubGet()) so post-generation package installs also have a timeout
• On timeout, the CLI now surfaces a clear error message instead of hanging indefinitely

Closes #1547

Context

When connected to a Wi-Fi network without internet access (e.g. phone hotspot with mobile data off), the bootstrapping step hangs indefinitely because the underlying dart pub get process never receives a socket error — it just waits forever. With no network at all, a socket error surfaces after ~35s, which is fine.

This PR adds timeouts at two points that can hang:

1. generator.hooks.preGen() in CreateSubCommand.runCreate — wrapped with .timeout(), catches TimeoutException, fails the progress indicator, logs a clear error, and returns ExitCode.unavailable
2. _Cmd.run() — accepts an optional Duration? timeout parameter; Dart.pubGet() and Flutter.pubGet() pass a 120s timeout so the process throws a ProcessException on timeout

Test plan

• Verify new test returns unavailable exit code when preGen times out passes in create_subcommand_test.dart
• Verify new test throws ProcessException when pub get times out passes in dart_cli_test.dart
• Verify new test throws ProcessException when pub get times out passes in flutter_cli_test.dart
• Verify all existing tests still pass
• Manual: connect to Wi-Fi with no internet, run very_good create flutter_app testapp — should fail with a timeout error after 120s instead of hanging indefinitely

https://claude.ai/code/session_01JoFEgFpRkH5idHrQFNy6xP

[marcossevilla]

lgtm but seems there's a few lint warnings, I can approve once CI passes @Statyk7