Stop throwing for not-found; add Guard and request validation

Not-found lookups return null/false instead of throwing NotFoundException
across all services — a missing row is expected control flow, not an
exceptional condition. NotFoundException stays for embedded precondition
checks inside mutations (missing parent, invalid foreign reference).

Guard (copied from mic-check) enforces required arguments at the top of
every service method. A ported IModelValidator<T> framework validates
every request DTO at the API layer via a new ValidationEndpointFilter,
returning a 400 with field-level messages; services re-run the same
validator and throw for direct callers that bypass the API.

Endpoints translate null/false into 404 via a new ToApiResult() helper.
The agent toolset boundary translates the same nullable/bool results
into the tool-error text the model already expected.

src/Novelly.Api/Agent/NovelAgentService.cs

@@ -4,6 +4,7 @@ using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 using Novelly.Api.Common;
+using Novelly.Api.Common.Validation;
 using Novelly.Api.Data;
 using Novelly.Api.Projects;
 
@@ -18,7 +19,8 @@ public class NovelAgentService(
     IAgentModelClient model,
     NovelAgentToolset toolset,
     IOptions<AgentOptions> options,
-    ILogger<NovelAgentService> logger)
+    ILogger<NovelAgentService> logger,
+    IModelValidator<SendAgentMessageRequest> sendMessageValidator)
 {
     private static readonly JsonSerializerOptions JsonOptions = new()
     {
@@ -39,11 +41,18 @@ public class NovelAgentService(
             .ToListAsync(ct);
     }
 
-    public async Task<ConversationDto> GetConversationAsync(Guid conversationId, CancellationToken ct = default)
+    /// <summary>Null when no conversation has this id — a lookup miss is expected, not exceptional.</summary>
+    public async Task<ConversationDto?> GetConversationAsync(Guid conversationId, CancellationToken ct = default)
     {
+        Guard.Default(conversationId, nameof(conversationId));
+
         logger.LogInformation("Getting agent conversation {ConversationId}", conversationId);
 
-        var conversation = await LoadConversationAsync(conversationId, ct);
+        var conversation = await FindConversationAsync(conversationId, ct);
+        if (conversation is null)
+        {
+            return null;
+        }
 
         return new ConversationDto(
             conversation.Id,
@@ -53,13 +62,22 @@ public class NovelAgentService(
             conversation.UpdatedAt);
     }
 
-    public async Task DeleteConversationAsync(Guid conversationId, CancellationToken ct = default)
+    /// <summary>True if a conversation was deleted; false if no conversation had this id.</summary>
+    public async Task<bool> DeleteConversationAsync(Guid conversationId, CancellationToken ct = default)
     {
+        Guard.Default(conversationId, nameof(conversationId));
+
         logger.LogInformation("Deleting agent conversation {ConversationId}", conversationId);
 
-        var conversation = await LoadConversationAsync(conversationId, ct);
+        var conversation = await FindConversationAsync(conversationId, ct);
+        if (conversation is null)
+        {
+            return false;
+        }
+
         db.Conversations.Remove(conversation);
         await db.SaveChangesAsync(ct);
+        return true;
     }
 
     /// <summary>
@@ -69,12 +87,19 @@ public class NovelAgentService(
     public async Task<AgentTurnDto> SendMessageAsync(
         Guid projectId, SendAgentMessageRequest request, CancellationToken ct = default)
     {
+        Guard.Default(projectId, nameof(projectId));
+        Guard.Null(request, nameof(request));
+        sendMessageValidator.Validate(request).ThrowIfInvalid();
+
         logger.LogInformation(
             "Sending agent message for project {ProjectId}, conversation {ConversationId}, message length {MessageLength}",
             projectId, request.ConversationId, request.Message.Length);
 
         var conversation = request.ConversationId is { } id
-            ? await LoadConversationAsync(id, ct)
+            ? await FindConversationAsync(id, ct)
+                // The id came from the request body, not the route — an unknown id here
+                // is bad input to this call, not a direct "fetch conversation" lookup.
+                ?? throw new NotFoundException(nameof(AgentConversation), id)
             : await StartConversationAsync(projectId, request.Message, ct);
 
         // Persist the user's turn before running the loop. The tools save through the
@@ -204,9 +229,9 @@ public class NovelAgentService(
         return conversation;
     }
 
-    private async Task<AgentConversation> LoadConversationAsync(Guid conversationId, CancellationToken ct)
+    private async Task<AgentConversation?> FindConversationAsync(Guid conversationId, CancellationToken ct)
     {
-        logger.LogDebug("Loading agent conversation {ConversationId}", conversationId);
+        logger.LogDebug("Finding agent conversation {ConversationId}", conversationId);
 
         var conversation = await db.Conversations
             .Include(c => c.Messages)
@@ -214,8 +239,7 @@ public class NovelAgentService(
 
         if (conversation is null)
         {
-            logger.LogWarning("AgentConversation {ConversationId} not found", conversationId);
-            throw new NotFoundException(nameof(AgentConversation), conversationId);
+            logger.LogInformation("AgentConversation {ConversationId} not found", conversationId);
         }
 
         return conversation;