Improve type specifications and documentation

- Added missing type specifications for Hello function and rules - Updated documentation for the Deserialize protocol -
Cleaned up IBAN validation function documentation - Enhanced test fixture generation with clearer parsing and error
messages

test/support/test_data.exs

@@ -214,7 +214,7 @@ defp filter_by_numeric_only(specs, numeric_only) do
     Enum.filter(specs, fn {_code, spec} ->
       # Use numeric_only field if available, otherwise fall back to bban_spec check
       case spec["numeric_only"] do
-        nil -> is_numeric_only?(spec["bban_spec"]) == numeric_only
+        nil -> numeric_only?(spec["bban_spec"]) == numeric_only
         value -> value == numeric_only
       end
     end)
@@ -230,7 +230,7 @@ defp has_national_check?(spec) do
     positions != nil and positions["start"] != positions["end"]
   end
 
-  defp is_numeric_only?(bban_spec) do
+  defp numeric_only?(bban_spec) do
     !String.contains?(bban_spec, ["!a", "!c"])
   end
 end