Test non-Latin script fallback in handle generation

Documents and pins the behaviour for scripts without transliteration
entries (Japanese, Chinese, Arabic, …):

- Fully non-Latin name (e.g. "日本語プロジェクト"): every initial
  maps to "?" via I18n.transliterate, filter_map drops them all,
  empty acronym falls back to FALLBACK_HANDLE ("PROJ").

- Mixed name (e.g. "Plan 日本"): Latin initials survive, non-Latin
  ones are silently dropped, result is the Latin-only acronym ("P").

spec/services/work_packages/project_handle_suggestion_generator_spec.rb

@@ -93,7 +93,11 @@ RSpec.describe WorkPackages::ProjectHandleSuggestionGenerator do
       "Flight Planning Training" => "FPT",
       "A B C D E F G H I J K" => "ABCDEFGHIJ", # truncated to 10 chars
       "Cécile Martin" => "CM", # Unicode: "Cécile" is one word, not ["C","cile"]
-      "étude de cas" => "EDC" # Unicode: é→E via transliteration
+      "étude de cas" => "EDC", # Unicode: é→E via transliteration
+      # Non-Latin scripts have no transliteration entries (I18n.transliterate → "?").
+      # All initials are dropped and the name falls back to FALLBACK_HANDLE.
+      "日本語プロジェクト" => "PROJ", # Japanese: every initial → "?" → fallback
+      "Plan 日本" => "P" # Mixed: Latin "P" survives; "日" is dropped
     }.each do |project_name, expected_handle|
       it "generates '#{expected_handle}' from '#{project_name}'" do
         create(:project, identifier: "bad-id", name: project_name)