Move InternalsVisibleTo attribute to project file

[Copilot]

Assembly-level attributes that control visibility should be declared in the project configuration rather than embedded in source files for better maintainability and clarity.

Changes:

• Moved InternalsVisibleTo attribute from src/DetectionDetail.cs to src/UTF-unknown.csproj
• Removed now-unnecessary System.Runtime.CompilerServices using directive

Before:

// src/DetectionDetail.cs
[assembly: InternalsVisibleTo("UtfUnknown.Tests, PublicKey=" +
"002400000480000094000000060200000024000052534131000400000100010029f6b4defac763...")]
namespace UtfUnknown;

After:

<!-- src/UTF-unknown.csproj -->
<ItemGroup>
  <InternalsVisibleTo Include="UtfUnknown.Tests, PublicKey=002400000480000094000000060200000024000052534131000400000100010029f6b4defac763..." />
</ItemGroup>

[Copilot]

Pull request overview

This PR moves the InternalsVisibleTo assembly-level attribute out of a source file and into the SDK-style project file, aligning visibility configuration with project configuration for easier maintenance and clearer ownership.

Changes:

• Added an <InternalsVisibleTo ... /> item in src/UTF-unknown.csproj for UtfUnknown.Tests (with the strong-name public key).
• Removed the [assembly: InternalsVisibleTo(...)] attribute from src/DetectionDetail.cs.
• Removed the now-unused System.Runtime.CompilerServices using directive from src/DetectionDetail.cs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/UTF-unknown.csproj	Adds InternalsVisibleTo as a project item so the SDK generates the assembly attribute during build.
src/DetectionDetail.cs	Removes the embedded assembly attribute and the unused System.Runtime.CompilerServices using.

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