Fix DEL character encoding in string literals

Change condition from `ch >= 128` to `ch >= 127` to include the DEL
character (U+007F, 127) in escaped output. This ensures that the
non-printable DEL control character is properly escaped as \u{7f}
instead of being printed directly.

Also add test cases for DEL and other control characters.

Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

src/test/zstring.cpp

@@ -21,6 +21,42 @@ static void tst_ascii_roundtrip() {
     }
 }
 
+// Test that control characters are properly escaped.
+static void tst_control_chars_escaped() {
+    // Test DEL character (0x7F / 127)
+    zstring del_char(0x7Fu);
+    std::string del_encoded = del_char.encode();
+    bool del_ok = del_encoded == "\\u{7f}";
+    
+    if (!del_ok) {
+        std::cout << "Failed to escape DEL character (0x7F): got '" << del_encoded 
+                  << "', expected '\\u{7f}'\n" << std::flush;
+        ENSURE(del_ok);
+    }
+    
+    // Test a few other control characters below 0x20
+    zstring null_char(0x00u);
+    std::string null_encoded = null_char.encode();
+    bool null_ok = null_encoded == "\\u{0}";
+    
+    if (!null_ok) {
+        std::cout << "Failed to escape NULL character (0x00): got '" << null_encoded 
+                  << "', expected '\\u{0}'\n" << std::flush;
+        ENSURE(null_ok);
+    }
+    
+    zstring tab_char(0x09u);
+    std::string tab_encoded = tab_char.encode();
+    bool tab_ok = tab_encoded == "\\u{9}";
+    
+    if (!tab_ok) {
+        std::cout << "Failed to escape TAB character (0x09): got '" << tab_encoded 
+                  << "', expected '\\u{9}'\n" << std::flush;
+        ENSURE(tab_ok);
+    }
+}
+
 void tst_zstring() {
     tst_ascii_roundtrip();
+    tst_control_chars_escaped();
 }

src/util/zstring.cpp

@@ -150,7 +150,7 @@ std::string zstring::encode() const {
 #define _flush() if (offset > 0) { buffer[offset] = 0; strm << buffer; offset = 0; }
     for (unsigned i = 0; i < m_buffer.size(); ++i) {
         unsigned ch = m_buffer[i];
-        if (ch < 32 || ch >= 128 || ('\\' == ch && i + 1 < m_buffer.size() && 'u' == m_buffer[i+1])) {
+        if (ch < 32 || ch >= 127 || ('\\' == ch && i + 1 < m_buffer.size() && 'u' == m_buffer[i+1])) {
             _flush();
             strm << "\\u{" << std::hex << ch << std::dec << '}';
         }