Fix importdirectory command description layout

[efegokdemir]

Summary

• Remove the embedded line break from the importdirectory command description so the shell formatter keeps the continuation aligned.
• Add a regression test covering the description output.

Testing

• mvn -pl shell -am -Dtest=ImportDirectoryCommandTest -DfailIfNoTests=false -DskipFormat test — passed with Java 21.
• mvn -pl shell -am -DskipTests -DverifyFormat verify — passed with Java 21.
• git diff --check — passed.

Fixes #6544

[utafrali]

The fix correctly removes the embedded newline and the regression test properly asserts against the actual newline character. One minor defect: replacing \n with a trailing space introduces a double space where the next concatenated string literal also begins with a space.

[utafrali]

The trailing space added here creates a double space when concatenated with the next string literal, which already starts with a space. The result is "...but no error is thrown. Bulk import only takes 2 arguments..." (two spaces before "Bulk"). Drop the trailing space since the following string provides the separator:

+ " will be displayed indicating the source directory is empty, but no error is thrown."