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.

tests/Novelly.Api.Tests/ListingTests.cs

@@ -99,7 +99,8 @@ public class ListingTests : ServiceTestFixture
             new ScriptedModelClient([[new AgentTextBlock("Reply.")]]),
             new NovelAgentToolset(Projects, Characters, Arcs, Chapters, Beats, Scenes, Tags, Questions, NullLogger<NovelAgentToolset>.Instance),
             Options.Create(new AgentOptions()),
-            NullLogger<NovelAgentService>.Instance);
+            NullLogger<NovelAgentService>.Instance,
+            new SendAgentMessageRequestValidator());
 
         await agent.SendMessageAsync(project.Id, new SendAgentMessageRequest("First question."));
         await agent.SendMessageAsync(project.Id, new SendAgentMessageRequest("Second question."));