Fix command-only code action serialization

[Arunsiva003]

What changed

• Represent CodeAction edit and command as undefined-or-present fields instead of nullable fields.
• Keep command-only code actions serializing without an edit property.
• Add a regression test for the sort imports code action wire shape.

Why it changed

Neovim treats edit: null as an invalid LSP code action response because edit is optional, not nullable. Omitting the field keeps the response aligned with the LSP shape while preserving the command.

Fixes #219

Tests run

• XDG_CACHE_HOME=/tmp /Users/arunkumar.a/.nvm/versions/node/v16.20.2/bin/spago test
• XDG_CACHE_HOME=/tmp npm run build