rdata_parsers: fix handling of escaped list delim

The RFC 9460 presentation syntax allows escaping the ',' list separator
in a `SvcParamValue` by writing `\,`. This commit updates `parse_list`
to handle this case.

Once this is done, one of the RFC 9460 test vectors using this feature
can be added to the unit tests. We don't yet support the more
complicated escaped comma using an escaped backslash for delimiter
escape so one remaining test vector for this is left out.

crates/proto/src/serialize/txt/rdata_parsers/svcb.rs

@@ -325,12 +325,36 @@ where
     T::Err: Into<ParseError>,
 {
     let mut result = Vec::new();
+    let mut current_value = String::new();
+    let mut escaping = false;
+
+    for c in value.chars() {
+        match (c, escaping) {
+            // End of value
+            (',', false) => {
+                result.push(T::from_str(&parse_char_data(&current_value)?).map_err(Into::into)?);
+                current_value.clear()
+            }
+            // Start of escape sequence
+            ('\\', false) => escaping = true,
+            // Comma inside escape sequence
+            (',', true) => {
+                current_value.push(',');
+                escaping = false
+            }
+            // Regular character inside escape sequence
+            (_, true) => {
+                current_value.push(c);
+                escaping = false
+            }
+            // Regular character
+            (_, false) => current_value.push(c),
+        }
+    }
 
-    let values = value.trim_end_matches(',').split(',');
-    for value in values {
-        let value = parse_char_data(value)?;
-        let value = T::from_str(&value).map_err(|e| e.into())?;
-        result.push(value);
+    // Push the remaining value if there's any
+    if !current_value.is_empty() {
+        result.push(T::from_str(&parse_char_data(&current_value)?).map_err(Into::into)?);
     }
 
     Ok(result)
@@ -452,7 +476,7 @@ mod tests {
 
         // NOTE: In each case the test vector from the RFC was augmented with a TTL (42 in each
         //       case). The parser requires this but the test vectors do not include it.
-        let vectors: [TestVector; 8] = [
+        let vectors: [TestVector; 9] = [
             // https://datatracker.ietf.org/doc/html/rfc9460#appendix-D.1
             // Figure 2: AliasMode
             TestVector {
@@ -553,8 +577,6 @@ mod tests {
                 ],
             },
             // Figure 10: An "alpn" Value with an Escaped Comma and an Escaped Backslash in Two Presentation Formats
-            /*
-             * TODO(XXX): Parser does not support escaped list delimiter.
             TestVector {
                 record: r#"example.com.  42  SVCB   16 foo.example.org. alpn="f\\\\oo\,bar,h2""#,
                 record_type: RecordType::SVCB,
@@ -565,7 +587,6 @@ mod tests {
                     SvcParamValue::Alpn(Alpn(vec![r#"f\\oo,bar"#.to_owned(), "h2".to_owned()])),
                 )],
             },
-             */
             /*
              * TODO(XXX): Parser does not replace escaped characters, does not see "\092," as
              *            an escaped delim.