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/NovelAgentServiceTests.cs

@@ -19,7 +19,8 @@ public class NovelAgentServiceTests : ServiceTestFixture
         model,
         _toolset,
         Options.Create(new AgentOptions { MaxIterations = 4 }),
-        NullLogger<NovelAgentService>.Instance);
+        NullLogger<NovelAgentService>.Instance,
+        new SendAgentMessageRequestValidator());
 
     [Test]
     public async Task A_plain_reply_is_persisted_as_a_conversation()
@@ -32,7 +33,7 @@ public class NovelAgentServiceTests : ServiceTestFixture
 
         Assert.That(turn.Message.Content, Is.EqualTo("Tell me about the ending."));
 
-        var conversation = await agent.GetConversationAsync(turn.ConversationId);
+        var conversation = (await agent.GetConversationAsync(turn.ConversationId))!;
 
         Assert.Multiple(() =>
         {
@@ -171,7 +172,7 @@ public class NovelAgentServiceTests : ServiceTestFixture
         var second = await agent.SendMessageAsync(
             projectId, new SendAgentMessageRequest("Question two.", first.ConversationId));
 
-        var conversation = await agent.GetConversationAsync(first.ConversationId);
+        var conversation = (await agent.GetConversationAsync(first.ConversationId))!;
 
         Assert.Multiple(() =>
         {