refactor(dotnet): dedupe has_nologo_arg/has_write_mode_override's shared predicate

Both hand-rolled the identical "Long flag, no attached value,
case-insensitive name match" shape (differing only in whether
double_dash is required), because neither arg_tokenizer.rs primitive
checks `attached` at all. Factored into dotnet_has_bare_loose_flag/
dotnet_has_bare_double_dash_flag, matching the existing
dotnet_has_loose_flag/dotnet_has_flag naming split.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>