fix(unparser): format values with parser ParsingCulture for round-trip

FormatCommandLine wrote option and value arguments using the ambient
thread culture via value.ToString(), while the parser reads them back
with ParserSettings.ParsingCulture (invariant by default). On threads
whose culture uses a comma decimal separator (for example de-DE) a
double 1.5 was unparsed as "1,5" and reparsed as 15, and a sequence
[1.5, 2.5] became "1,5,2,5" and reparsed as four elements [1,5,2,5],
so the format/parse round-trip was broken.

Format IFormattable values (numeric and date/time types) with the
parser ParsingCulture instead of the current thread culture, for both
scalar and separator-joined sequence values. Other types keep their
default string representation, so behaviour on invariant/en-US is
unchanged.

Add round-trip tests under de-DE for a scalar double and a
comma-separated double sequence.

tests/CommandLine.Tests/Unit/UnParserExtensionsTests.cs

@@ -2,8 +2,10 @@
 
 using System;
 using System.Collections.Generic;
+using System.Globalization;
 using System.IO;
 using System.Linq;
+using System.Threading;
 using Xunit;
 using FluentAssertions;
 using CommandLine.Tests.Fakes;
@@ -154,6 +156,66 @@ namespace CommandLine.Tests.Unit
         }
         #endregion
 
+        #region Parsing culture round-trip
+
+        [Fact]
+        public static void UnParsing_scalar_double_uses_parsing_culture_so_it_round_trips_on_comma_decimal_cultures()
+        {
+            var originalCulture = Thread.CurrentThread.CurrentCulture;
+            try
+            {
+                // A culture that uses ',' as the decimal separator (e.g. de-DE) would,
+                // when the ambient thread culture is used, format 1.5 as "1,5", which the
+                // invariant-culture parser reparses as 15 - breaking the round-trip.
+                Thread.CurrentThread.CurrentCulture = new CultureInfo("de-DE");
+                var parser = new Parser();
+                var options = new Simple_Options_With_Double_Value { DoubleValue = 1.5 };
+
+                var commandLine = parser.FormatCommandLine(options);
+
+                commandLine.Should().Contain("1.5");
+                commandLine.Should().NotContain("1,5");
+
+                var reparsed = parser.ParseArguments<Simple_Options_With_Double_Value>(commandLine.SplitArgs());
+                reparsed.Tag.Should().Be(ParserResultType.Parsed);
+                ((Parsed<Simple_Options_With_Double_Value>)reparsed).Value.DoubleValue.Should().Be(1.5);
+            }
+            finally
+            {
+                Thread.CurrentThread.CurrentCulture = originalCulture;
+            }
+        }
+
+        [Fact]
+        public static void UnParsing_double_sequence_uses_parsing_culture_so_it_round_trips_on_comma_decimal_cultures()
+        {
+            var originalCulture = Thread.CurrentThread.CurrentCulture;
+            try
+            {
+                // With a comma-decimal ambient culture each element would be formatted as
+                // "1,5"/"2,5"; joined by the ',' sequence separator this yields "1,5,2,5",
+                // which the parser reparses as four elements [1,5,2,5] instead of [1.5,2.5].
+                Thread.CurrentThread.CurrentCulture = new CultureInfo("de-DE");
+                var parser = new Parser();
+                var options = new Options_With_Double_Sequence { DoubleSequence = new[] { 1.5, 2.5 } };
+
+                var commandLine = parser.FormatCommandLine(options);
+
+                commandLine.Should().Be("--dbls 1.5,2.5");
+
+                var reparsed = parser.ParseArguments<Options_With_Double_Sequence>(commandLine.SplitArgs());
+                reparsed.Tag.Should().Be(ParserResultType.Parsed);
+                ((Parsed<Options_With_Double_Sequence>)reparsed).Value.DoubleSequence
+                    .Should().Equal(new[] { 1.5, 2.5 });
+            }
+            finally
+            {
+                Thread.CurrentThread.CurrentCulture = originalCulture;
+            }
+        }
+
+        #endregion
+
         #region PR 550
 
         [Fact]