Expand unit coverage for core records and extension helpers

[Copilot]

Test coverage was thin around foundational value types and extension helpers used by client/server flows. This adds focused unit coverage to close gaps and guard key behaviors.

• Value types & record behavior
  • Cover Message/Payload/Metadata constructors, flags, and equality
  • Validate record equality for ServerMessage and ErrorOccurred
• Internal and public extension helpers
  • Exercise Internal.Extensions.Try/Async/GetMetadata and ToPayload encoding
  • Validate client/server observable send/broadcast extension routing via test doubles

Example coverage:

var message = Message.Create("Hello");
Assert.True(message.IsText);
Assert.Equal("Hello", message.ToString());

using var payload = "Hello".AsMemory().ToPayload(Encoding.UTF8, WebSocketMessageType.Text);
Assert.Equal(Encoding.UTF8.GetBytes("Hello"), payload.Data.ToArray());