Code(API::CreateAssistant): Implement create assistant feature

src/Server/StellarChat.Server.Api/Features/Assistants/CreateAssistant/CreateAssistant.cs

@@ -0,0 +1,11 @@
+namespace StellarChat.Server.Api.Features.Assistants.CreateAssistant;
+
+internal sealed record CreateAssistant(
+    [Required] Guid Id,
+    string Name,
+    string Metaprompt,
+    string Description,
+    string AvatarUrl,
+    string DefaultModel,
+    string DefaultVoice,
+    bool IsDefault) : ICommand;

src/Server/StellarChat.Server.Api/Features/Assistants/CreateAssistant/CreateAssistantEndpoint.cs

@@ -0,0 +1,32 @@
+using StellarChat.Server.Api.Features.Chat.CreateChatSession;
+
+namespace StellarChat.Server.Api.Features.Assistants.CreateAssistant;
+
+internal sealed class CreateAssistantEndpoint : IEndpoint 
+{
+    public void Expose(IEndpointRouteBuilder endpoints)
+    {
+        var assistants = endpoints.MapGroup("/assistants").WithTags("Assistants");
+
+        assistants.MapPost("", async ([FromBody] CreateAssistantRequest request, IMediator mediator) =>
+        {
+            var id = Guid.NewGuid();
+            var command = request.Adapt<CreateAssistant>();
+
+            command = command with { Id = id };
+            await mediator.Send(command);
+
+            return Results.CreatedAtRoute("GetAssistant", new { Id = id }, id);
+        })
+         .Produces(StatusCodes.Status201Created)
+         .Produces(StatusCodes.Status400BadRequest)
+         .WithOpenApi(operation => new(operation)
+         {
+             Summary = "Creates a new assistant."
+         });
+    }
+
+    public void Register(IServiceCollection services, IConfiguration configuration) { }
+
+    public void Use(IApplicationBuilder app) { }
+}

src/Server/StellarChat.Server.Api/Features/Assistants/CreateAssistant/CreateAssistantHandler.cs

@@ -0,0 +1,60 @@
+namespace StellarChat.Server.Api.Features.Assistants.CreateAssistant;
+
+internal sealed class CreateAssistantHandler : ICommandHandler<CreateAssistant>
+{
+    private readonly IAssistantRepository _assistantRepository;
+    private readonly TimeProvider _clock;
+    private readonly ILogger<CreateAssistantHandler> _logger;
+
+    public CreateAssistantHandler(IAssistantRepository assistantRepository, TimeProvider clock, ILogger<CreateAssistantHandler> logger)
+    {
+        _assistantRepository = assistantRepository;
+        _clock = clock;
+        _logger = logger;
+    }
+
+    public async ValueTask<Unit> Handle(CreateAssistant command, CancellationToken cancellationToken)
+    {
+        if (await _assistantRepository.ExistsAsync(command.Id))
+        {
+            throw new AssistantAlreadyExistsException(command.Id);
+        }
+
+        var now = _clock.GetUtcNow();
+
+        if (command.IsDefault)
+        {
+            await UnsetPreviousDefaultAssistant();
+        }
+
+        var assistant = Assistant.Create(
+            command.Id,
+            command.Name,
+            command.Metaprompt,
+            command.Description,
+            command.AvatarUrl,
+            command.DefaultModel,
+            command.DefaultVoice,
+            command.IsDefault,
+            createdAt: now,
+            updatedAt: now);
+
+        await _assistantRepository.AddAsync(assistant);
+        _logger.LogInformation($"Assistant with ID: '{assistant.Id}' has been created.");
+
+        return Unit.Value;
+    }
+
+    private async Task UnsetPreviousDefaultAssistant()
+    {
+        var assistants = await _assistantRepository.BrowseAsync();
+        var currentDefaultAssistant = assistants.SingleOrDefault(assistant => assistant.IsDefault);
+
+        if (currentDefaultAssistant is not null)
+        {
+            currentDefaultAssistant.IsDefault = false;
+            await _assistantRepository.UpdateAsync(currentDefaultAssistant);
+        }
+    }
+}
+