Fix ClearSign UTF-8 encoding corruption

[Copilot]

The ClearSign methods were using ASCII encoding to convert text to bytes, corrupting non-ASCII characters (š → ?, € → ?, 中文 → ???).

Changes

• OutputClearSigned and OutputClearSignedAsync: Changed Encoding.ASCII.GetBytes() to Encoding.UTF8.GetBytes() for line content conversion
• Tests: Added comprehensive UTF-8 character preservation tests covering all ClearSign method variants:
  • String methods (sync and async)
  • File methods (sync and async)
  • Stream methods (sync and async)
  • Tests cover Latin extended, currency symbols, and CJK characters with 3 key types each (Generated, Known, KnownGpg)
• Test framework: Updated from .NET 6.0 to .NET 8.0 target

Example

var pgp = new PGP(encryptionKeys);
string content = "Message with UTF-8: š ž € 中文";
string signed = pgp.ClearSign(content);
// Before: "Message with UTF-8: ? ? ? ??"
// After:  "Message with UTF-8: š ž € 中文" ✓

The change affects only the encoding step in clear-signing; signature generation and verification remain unchanged.

Testing

• ✅ All 153 ClearSign tests pass (including 18 UTF-8 encoding tests)
• ✅ UTF-8 preservation validated across all method variants (string, file, stream)
• ✅ Compatible with both .NET Framework 4.7.2 and .NET 8.0

Original prompt

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This section details on the original issue you should resolve

<issue_title>Clearsign converts UTF8 content to ASCII</issue_title>
<issue_description>Hello @mattosaurus thank you for such a useful package!

There is one minor problem I've faced during using the Clearsign method:

    private void OutputClearSigned(Stream inputStream, Stream outputStream, IDictionary<string, string> headers)
    {
        using StreamReader streamReader = new StreamReader(inputStream);
        using ArmoredOutputStream armoredOutputStream = new ArmoredOutputStream(outputStream, headers);
        PgpSignatureGenerator pgpSignatureGenerator = InitClearSignatureGenerator(armoredOutputStream);
        while (streamReader.Peek() >= 0)
        {
            string text = streamReader.ReadLine();
            if (text != null)
            {
                byte[] bytes = Encoding.ASCII.GetBytes(text);
                string s = text.TrimEnd(Array.Empty<char>());
                byte[] bytes2 = Encoding.ASCII.GetBytes(s);
                pgpSignatureGenerator.Update(bytes2, 0, bytes2.Length);
                armoredOutputStream.Write(bytes, 0, bytes.Length);
                armoredOutputStream.Write(13);
                armoredOutputStream.Write(10);
                if (streamReader.Peek() >= 0)
                {
                    pgpSignatureGenerator.Update(13);
                    pgpSignatureGenerator.Update(10);
                }
            }
        }

        armoredOutputStream.EndClearText();
        BcpgOutputStream outStream = new BcpgOutputStream(armoredOutputStream);
        pgpSignatureGenerator.Generate().Encode(outStream);
    }		

It's working completely fine but I'm using the UTF8 symbols, in those methods data is being converted to the ASCII, that breaks those symbols (e.g. š -> ?)

byte[] lineByteArray = Encoding.ASCII.GetBytes(line);
byte[] cleanLineByteArray = Encoding.ASCII.GetBytes(cleanLine);

Would be really cool to have an opportunity using UTF8 as well, if you don't mind some help, I could try to assist with that

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Clearsign converts UTF8 content to ASCII #320

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[mattosaurus]

@copilot update to also add tests for the sync and async stream and file methods in addition to the existing string tests for verifying the encoding.

[Copilot]

@copilot update to also add tests for the sync and async stream and file methods in addition to the existing string tests for verifying the encoding.

Added UTF-8 encoding tests for all ClearSign method variants (file and stream, both sync and async). All 18 new tests pass successfully. Commit: 5e10e01

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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