Merge pull request #1 from suztomo/librarian-20250908T194747Z

Librarian release pull request: 20250908T194747Z

fix(sidekick): skip dart directory (googleapis#1570)

For googleapis#1563

fix(internal/librarian): don't display both usage twice (googleapis#668)

If Command.Parse fails having found a specific command, we can expect
that command to have displayed a suitable command-specific-usage
message; we don't need to display the generic usage as well.

This affects both the case of "help explicitly requested with -h" as
well as "a flag that's not registed for this command was supplied".

Fixes googleapis#666