Insert trailing space after keyword and annotation completions

jdt.ls recently added a fix (eclipse-jdtls/eclipse.jdt.ls#3868) that inserts a trailing space after certain keyword completions (public, static, void, etc.) and annotation completions (like @Deprecated), so typing can continue smoothly without manually adding a space. This matches IntelliJ's behavior and was requested in redhat-developer/vscode-java#4401.

Right now, the completion text jdt.core returns for these doesn't include the trailing space — jdt.ls has to add it itself on the client side. It might make more sense for jdt.core's completion engine to include it natively, so any client benefits, not just jdt.ls.

Start by tracing the jdt.core completion engine paths that produce keyword and annotation completion text, using the jdt.ls fix eclipse-jdtls/eclipse.jdt.ls#3868 as behavioral context. Verify that completions such as public, static, void, and @Deprecated include a trailing space, and add or update completion tests to confirm the returned text.

Written by the indexing model from the issue text.