Support field separators on the last field of a message field.

https://protobuf.dev/reference/protobuf/textformat-spec/#fields calls out that
`MessageField` and `ScalarField` both can be followed by a separator (`,` or
`;`).

https://protobuf.dev/reference/protobuf/textformat-spec/#message then replies on
this to handle the separators within a message field.

The existing parsing was accepting separators after all fields for top level
fields, but when the decoder was for a message field (i.e. - there is a
terminator stop at), it wouldn't properly the final separator followed by
terminator correctly.

Author: thomasvl

Sources/SwiftProtobuf/TextFormatDecoder.swift

@@ -63,20 +63,24 @@ internal struct TextFormatDecoder: Decoder {
     }
 
     mutating func nextFieldNumber() throws -> Int? {
-        if let terminator = terminator {
-            if scanner.skipOptionalObjectEnd(terminator) {
-                return nil
-            }
-        }
+        // Per https://protobuf.dev/reference/protobuf/textformat-spec/#fields, every field can be
+        // followed by a field separator, so if we've seen a field, remove the separator before
+        // checking for the terminator.
         if fieldCount > 0 {
             scanner.skipOptionalSeparator()
         }
+        if let terminator = terminator,
+           scanner.skipOptionalObjectEnd(terminator) {
+            return nil
+        }
         if let fieldNumber = try scanner.nextFieldNumber(names: fieldNameMap!, messageType: messageType) {
             fieldCount += 1
             return fieldNumber
         } else if terminator == nil {
             return nil
         } else {
+            // If this decoder is looking for at a terminator, then if the scanner failed to
+            // find a field number, something went wrong (end of stream).
             throw TextFormatDecodingError.truncated
         }
 

Tests/SwiftProtobufTests/Test_TextFormat_proto3.swift

@@ -1298,6 +1298,12 @@ final class Test_TextFormat_proto3: XCTestCase, PBTestHelpers {
         assertTextFormatDecodeSucceeds("optional_int32:1;\n") {(o: MessageTestType) in
             return o.optionalInt32 == 1
         }
+        assertTextFormatDecodeSucceeds("optional_nested_message {bb:3,},") {(o: MessageTestType) in
+            return o.optionalNestedMessage.bb == 3
+        }
+        assertTextFormatDecodeSucceeds("optional_nested_message {bb:7;};") {(o: MessageTestType) in
+            return o.optionalNestedMessage.bb == 7
+        }
     }
 
     //