Make LengthPrefixedBlock fields readonly

[Copilot]

The LengthPrefixedBlock struct had mutable public fields that could be modified after creation, violating immutability best practices for value types used as tokens.

Changes

• Converted LengthPrefixedBlock to use primary constructor syntax, making fields readonly by default
• Updated instantiation site to use constructor instead of object initializer

Before

public record struct LengthPrefixedBlock {
    public int LengthPosition;
    public int ContentPosition;
}

// Usage
return new LengthPrefixedBlock {
    LengthPosition = lengthPosition,
    ContentPosition = contentPosition
};

After

public record struct LengthPrefixedBlock(int LengthPosition, int ContentPosition);

// Usage
return new LengthPrefixedBlock(lengthPosition, contentPosition);

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.