fix: use codepoint count for std.format width padding

Motivation:
`std.format("%5s", "😀")` produced 3 spaces + emoji because width was
computed using String.length (UTF-16 code units), counting the surrogate
pair as 2. go-jsonnet and Python both count codepoints, yielding 4 spaces.

Modification:
Replace `.length` with `.codePointCount(0, _.length)` in the `widen`
function's missingWidth calculation. This is consistent with the precision
truncation in the same file which already uses codePointCount.

Result:
Width padding now counts supplementary characters as 1 codepoint, matching
go-jsonnet and Python behavior. ASCII-only strings (the common case) are
unaffected since codePointCount == length for BMP text.

Author: He-Pin

sjsonnet/src/sjsonnet/Format.scala

@@ -286,7 +286,10 @@ object Format {
       else if (signedConversion && formatted.signCharacter) "+" + lhs
       else lhs
 
-    val missingWidth = formatted.widthOr(-1) - lhs2.length - mhs.length - rhs.length
+    val missingWidth = formatted.widthOr(-1) -
+      lhs2.codePointCount(0, lhs2.length) -
+      mhs.codePointCount(0, mhs.length) -
+      rhs.codePointCount(0, rhs.length)
 
     if (missingWidth <= 0) {
       // Avoid unnecessary string concatenation when parts are empty

sjsonnet/test/resources/new_test_suite/format_width_codepoint.jsonnet

@@ -0,0 +1,11 @@
+// Width padding must count codepoints, not UTF-16 code units.
+// Supplementary characters (surrogate pairs) count as 1 codepoint.
+[
+  std.assertEqual("%5s" % "😀", "    😀"),
+  std.assertEqual("%-5s" % "😀", "😀    "),
+  std.assertEqual("%5s" % "a😀b", "  a😀b"),
+  std.assertEqual("%3s" % "😀😀", " 😀😀"),
+  std.assertEqual("%5.1s" % "😀x", "    😀"),
+  std.assertEqual("%5s" % "hello", "hello"),
+  std.assertEqual("%10s" % "hello", "     hello"),
+]

sjsonnet/test/resources/new_test_suite/format_width_codepoint.jsonnet.golden

@@ -0,0 +1,9 @@
+[
+   true,
+   true,
+   true,
+   true,
+   true,
+   true,
+   true
+]