test(stdlib): add coverage for untested builtins - abs, chr/ord, len, map, type conversions, isinstance, bytes

Adds 15 new [<Fact>] tests for builtins that were bound in Builtins.fs
but had no test coverage:
- abs (int and float)
- chr / ord (and round-trip)
- len (list and string)
- map (single and two-iterable overloads)
- str / int / float type conversions
- isinstance / type
- bytes (from byte array and from string with encoding)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

CHANGELOG.md

@@ -7,6 +7,12 @@ name: Fable.Python
 
 All notable changes to this project will be documented in this file.
 
+## Unreleased
+
+### 🧪 Tests
+
+* *(stdlib)* Add tests for untested builtins: `abs`, `chr`/`ord`, `len`, `map`, type conversions (`str`/`int`/`float`), `isinstance`, `type`, `bytes`
+
 ## 5.1.0 - 2026-04-26
 
 ### 🚀 Features

test/TestBuiltins.fs

@@ -65,3 +65,89 @@ let ``test any works`` () =
     builtins.any [ false; false; true ] |> equal true
     builtins.any [ false; false; false ] |> equal false
     builtins.any [] |> equal false
+
+[<Fact>]
+let ``test abs with int works`` () =
+    builtins.abs -5 |> equal 5
+    builtins.abs 0 |> equal 0
+    builtins.abs 42 |> equal 42
+
+[<Fact>]
+let ``test abs with float works`` () =
+    builtins.abs -3.14 |> equal 3.14
+    builtins.abs 0.0 |> equal 0.0
+    builtins.abs 2.72 |> equal 2.72
+
+[<Fact>]
+let ``test chr and ord round-trip works`` () =
+    builtins.chr 65 |> equal 'A'
+    builtins.chr 97 |> equal 'a'
+    builtins.ord 'A' |> equal 65
+    builtins.ord 'a' |> equal 97
+
+[<Fact>]
+let ``test chr ord round-trip preserves value`` () =
+    let code = 9731 // snowman ☃
+    builtins.ord (builtins.chr code) |> equal code
+
+[<Fact>]
+let ``test len with list works`` () =
+    builtins.len ([ 1; 2; 3 ] |> box) |> equal 3
+    builtins.len ([] |> box) |> equal 0
+
+[<Fact>]
+let ``test len with string works`` () =
+    builtins.len ("hello" |> box) |> equal 5
+    builtins.len ("" |> box) |> equal 0
+
+[<Fact>]
+let ``test map with single iterable works`` () =
+    builtins.map ((fun x -> x * 2), [ 1; 2; 3 ])
+    |> Seq.toList
+    |> equal [ 2; 4; 6 ]
+
+[<Fact>]
+let ``test map with two iterables works`` () =
+    builtins.map ((fun (a, b) -> a + b), [ 1; 2; 3 ], [ 10; 20; 30 ])
+    |> Seq.toList
+    |> equal [ 11; 22; 33 ]
+
+[<Fact>]
+let ``test str conversion works`` () =
+    builtins.str 42 |> equal "42"
+    builtins.str true |> equal "True"
+
+[<Fact>]
+let ``test int conversion works`` () =
+    builtins.int "42" |> equal 42
+    builtins.int 3.9 |> equal 3
+
+[<Fact>]
+let ``test float conversion works`` () =
+    builtins.float "3.14" |> equal 3.14
+    builtins.float 42 |> equal 42.0
+
+[<Fact>]
+let ``test isinstance works`` () =
+    builtins.isinstance ("hello", builtins.``type`` "hello") |> equal true
+    builtins.isinstance (42, builtins.``type`` 42) |> equal true
+
+[<Fact>]
+let ``test isinstance returns false for wrong type`` () =
+    builtins.isinstance (42, builtins.``type`` "hello") |> equal false
+
+[<Fact>]
+let ``test type returns type object`` () =
+    let t = builtins.``type`` "hello"
+    builtins.isinstance ("world", t) |> equal true
+    builtins.isinstance (42, t) |> equal false
+
+[<Fact>]
+let ``test bytes from byte array works`` () =
+    let b = builtins.bytes [| 72uy; 101uy; 108uy; 108uy; 111uy |]
+    builtins.len b |> equal 5
+
+[<Fact>]
+let ``test bytes from string with encoding works`` () =
+    let b = builtins.bytes ("ABC", "utf-8")
+    builtins.len b |> equal 3