Add inline frame format helpers to C# tests

[Copilot]

C# tests failed to compile after #112 removed legacy compatibility layers. The commit added inline frame format helpers to C, C++, TypeScript, and JavaScript tests but missed C#.

Changes

• Added Fletcher-16 checksum helper - Internal static method for CRC validation
• Implemented 6 frame format helper classes - BasicDefault, TinyMinimal, BasicExtended, BasicExtendedMultiSystemStream, BasicMinimal, TinyDefault extending FrameFormatBase
• Created TestCodecHelpers static class - Factory pattern for parser instantiation, encoding, and decoding
• Updated test runner - Changed references from TestCodec to TestCodecHelpers

Each helper class implements the Encode() and ValidatePacket() methods following the same pattern as other languages:

class BasicDefault : FrameFormatBase
{
    public override byte[] Encode(int msgId, byte[] msgData)
    {
        const int headerSize = 4;
        const int footerSize = 2;
        byte[] buffer = new byte[headerSize + msgData.Length + footerSize];
        buffer[0] = HeaderConstants.BASIC_START_BYTE;
        buffer[1] = HeaderBasic.GetSecondStartByte(1); // DEFAULT = 1
        buffer[2] = (byte)msgData.Length;
        buffer[3] = (byte)msgId;
        // ... payload and CRC
    }
}

Pattern matches C, C++, TypeScript, and JavaScript implementations for consistency.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fix failing tests </issue_title>
<issue_description></issue_description>

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

• Fixes Fix failing tests  #113

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