fix: std.format %c rejects multi-character strings

sjsonnet/src/sjsonnet/Format.scala

@@ -696,6 +696,15 @@ object Format {
                 Error.fail(
                   "expected number at position %d, got string".format(i)
                 )
+              if (formatted.conversion == 'c') {
+                val s = vs.str
+                val cpCount = s.codePointCount(0, s.length)
+                if (cpCount != 1)
+                  Error.fail(
+                    "%%c expected 1-sized string got: %d".format(cpCount),
+                    pos
+                  )
+              }
               widenRaw(formatted, vs.str)
             case vn: Val.Num =>
               val s = vn.asDouble

sjsonnet/test/resources/new_test_suite/error.format_c_multi_char_string.jsonnet

@@ -0,0 +1 @@
+"%c" % "AB"

sjsonnet/test/resources/new_test_suite/error.format_c_multi_char_string.jsonnet.golden

@@ -0,0 +1,2 @@
+sjsonnet.Error: [std.format] %c expected 1-sized string got: 2
+    at [<root>].(error.format_c_multi_char_string.jsonnet:1:6)

sjsonnet/test/resources/new_test_suite/format_c_single_char.jsonnet

@@ -0,0 +1,8 @@
+// Regression tests for std.format %c — verified against C++ jsonnet v0.21.0
+// Success cases: single-char strings and numeric codepoints
+std.assertEqual("%c" % "A", "A") &&
+std.assertEqual("%c" % 65, "A") &&
+std.assertEqual("%c" % "世", "世") &&
+std.assertEqual("%c" % 127757, "🌍") &&
+std.assertEqual("%10c" % "A", "         A") &&
+true

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

@@ -0,0 +1 @@
+true