Update spfx create help snapshot for optional --library-name

[Copilot]

The spfx create help snapshot still reflected --library-name as required after the flag was made optional. This updates the CLI help snapshot to match the current command contract and help text.

• Snapshot alignment

  • Updated CommandLineHelp.test.ts.snap for spfx create so the usage output shows --library-name as optional.
  • Updated the parameter description to reflect the fallback behavior to the solution name.

• Scope

  • No production code changes.
  • No behavior changes beyond bringing the recorded help output in sync with the existing implementation.

Before:
--template TEMPLATE_NAME --library-name LIBRARY_NAME --component-name COMPONENT_NAME

After:
--template TEMPLATE_NAME [--library-name LIBRARY_NAME] --component-name COMPONENT_NAME