fix: encode non-BMP characters as UTF-16 surrogate pairs in AsciiJSON

The AsciiJSON Render method used fmt.Appendf(buf, "\u%04x", r) for all
non-ASCII runes. For non-BMP characters (code points > 0xFFFF, e.g. emoji),
this produced invalid JSON with raw 5+ hex digit sequences like \u1f389.

Fix: use utf16.Encode([]rune{r}) to convert non-BMP runes into UTF-16
surrogate pairs, producing valid JSON like \ud83c\udf89.

Fixes #4688

Signed-off-by: Md Mushfiqur Rahim <20mahin2020@gmail.com>

render/json.go

@@ -10,6 +10,7 @@ import (
 	"html/template"
 	"net/http"
 	"unicode"
+	"unicode/utf16"
 
 	"github.com/gin-gonic/gin/codec/json"
 	"github.com/gin-gonic/gin/internal/bytesconv"
@@ -160,11 +161,16 @@ func (r AsciiJSON) Render(w http.ResponseWriter) error {
 	}
 
 	var buffer bytes.Buffer
-	escapeBuf := make([]byte, 0, 6) // Preallocate 6 bytes for Unicode escape sequences
+	escapeBuf := make([]byte, 0, 12) // Preallocate for up to two \uXXXX sequences
 
 	for _, r := range bytesconv.BytesToString(ret) {
 		if r > unicode.MaxASCII {
-			escapeBuf = fmt.Appendf(escapeBuf[:0], "\\u%04x", r) // Reuse escapeBuf
+			if r > 0xFFFF {
+				surrogates := utf16.Encode([]rune{r})
+				escapeBuf = fmt.Appendf(escapeBuf[:0], "\\u%04x\\u%04x", surrogates[0], surrogates[1])
+			} else {
+				escapeBuf = fmt.Appendf(escapeBuf[:0], "\\u%04x", r)
+			}
 			buffer.Write(escapeBuf)
 		} else {
 			buffer.WriteByte(byte(r))