fix(beam): handle string binaries in Seq enumerator (#4591)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dbrattli

src/fable-library-beam/fable_utils.erl

@@ -153,6 +153,9 @@ get_enumerator(Map) when is_map(Map) ->
 get_enumerator({group_collection, Items, _Names}) ->
     %% Regex GroupCollection: iterate over the group items list
     get_enumerator(Items);
+get_enumerator(Bin) when is_binary(Bin) ->
+    %% String binary used as seq<char>: enumerate Unicode codepoints.
+    get_enumerator(unicode:characters_to_list(Bin));
 get_enumerator(Other) ->
     %% Fallback: treat as list
     get_enumerator(lists:flatten([Other])).

tests/Beam/StringTests.fs

@@ -1055,3 +1055,25 @@ let ``test String.forall works`` () =
     "aaa" |> String.forall (fun c -> c = 'a') |> equal true
     "aab" |> String.forall (fun c -> c = 'a') |> equal false
     "" |> String.forall (fun c -> c = 'a') |> equal true
+
+[<Fact>]
+let ``test Seq.mapi over string works`` () =
+    "Hello"
+    |> Seq.mapi (fun _ c -> string c)
+    |> String.concat ""
+    |> equal "Hello"
+
+[<Fact>]
+let ``test Seq.map over string works`` () =
+    "abc"
+    |> Seq.map (fun c -> string c)
+    |> String.concat "-"
+    |> equal "a-b-c"
+
+[<Fact>]
+let ``test Seq.toList over string works`` () =
+    "abc" |> Seq.toList |> equal [ 'a'; 'b'; 'c' ]
+
+[<Fact>]
+let ``test Seq.length over string works`` () =
+    "Hello" |> Seq.length |> equal 5