Implement NewType pattern for VoiceModelId to improve type safety

[Copilot]

This PR implements a NewType pattern (similar to Rust's NewType or Scala's OpaqueType) for model IDs in the VoicevoxCoreSharp library, making the types more clear and ensuring better type safety.

Changes

• Created a VoiceModelId record type to wrap string IDs
• Updated VoiceModelFile to use VoiceModelId instead of raw strings
• Modified Synthesizer methods to accept and work with VoiceModelId
• Added backward compatibility methods to ensure existing code continues to work
• Updated tests to handle the new type system

Benefits

• Type Safety: IDs are now strongly-typed instead of being generic strings
• API Clarity: Method signatures clearly indicate what type of ID is expected
• Error Prevention: Compile-time checking prevents using the wrong type of ID
• Backward Compatibility: Existing code continues to work through implicit conversions and overloaded methods

Implementation Details

// The new record type
public record VoiceModelId(string Id)
{
    // Target framework is netstandard2.0, so init-only properties not available
    public string Id { get; } = Id;

    public static implicit operator string(VoiceModelId id) => id.Id;
    
    public static implicit operator VoiceModelId(string id) => new VoiceModelId(id);
    
    public override string ToString() => Id;
}

Fixes #206.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• cdn.fwupd.org
  • Triggering command: /usr/bin/fwupdmgr refresh (dns block)
• crl3.digicert.com
  • Triggering command: dotnet build src/VoicevoxCoreSharp.Core/VoicevoxCoreSharp.Core.csproj (dns block)
  • Triggering command: dotnet test tests/VoicevoxCoreSharp.Core.Tests/VoicevoxCoreSharp.Core.Tests.csproj --filter FullyQualifiedName~VoiceModelTest (dns block)
• crl4.digicert.com
  • Triggering command: dotnet build src/VoicevoxCoreSharp.Core/VoicevoxCoreSharp.Core.csproj (dns block)
  • Triggering command: dotnet test tests/VoicevoxCoreSharp.Core.Tests/VoicevoxCoreSharp.Core.Tests.csproj --filter FullyQualifiedName~VoiceModelTest (dns block)
• ocsp.digicert.com
  • Triggering command: dotnet build src/VoicevoxCoreSharp.Core/VoicevoxCoreSharp.Core.csproj (dns block)
  • Triggering command: dotnet test tests/VoicevoxCoreSharp.Core.Tests/VoicevoxCoreSharp.Core.Tests.csproj --filter FullyQualifiedName~VoiceModelTest (dns block)
• s.symcb.com
  • Triggering command: dotnet build src/VoicevoxCoreSharp.Core/VoicevoxCoreSharp.Core.csproj (dns block)
  • Triggering command: dotnet test tests/VoicevoxCoreSharp.Core.Tests/VoicevoxCoreSharp.Core.Tests.csproj --filter FullyQualifiedName~VoiceModelTest (dns block)
• s.symcd.com
  • Triggering command: dotnet build src/VoicevoxCoreSharp.Core/VoicevoxCoreSharp.Core.csproj (dns block)
  • Triggering command: dotnet test tests/VoicevoxCoreSharp.Core.Tests/VoicevoxCoreSharp.Core.Tests.csproj --filter FullyQualifiedName~VoiceModelTest (dns block)
• ts-crl.ws.symantec.com
  • Triggering command: dotnet build src/VoicevoxCoreSharp.Core/VoicevoxCoreSharp.Core.csproj (dns block)
  • Triggering command: dotnet test tests/VoicevoxCoreSharp.Core.Tests/VoicevoxCoreSharp.Core.Tests.csproj --filter FullyQualifiedName~VoiceModelTest (dns block)
• ts-ocsp.ws.symantec.com
  • Triggering command: dotnet build src/VoicevoxCoreSharp.Core/VoicevoxCoreSharp.Core.csproj (dns block)
  • Triggering command: dotnet test tests/VoicevoxCoreSharp.Core.Tests/VoicevoxCoreSharp.Core.Tests.csproj --filter FullyQualifiedName~VoiceModelTest (dns block)
• www.microsoft.com
  • Triggering command: dotnet test tests/VoicevoxCoreSharp.Core.Tests/VoicevoxCoreSharp.Core.Tests.csproj --filter FullyQualifiedName~VoiceModelTest (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

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