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/LoggingTests.cs

@@ -15,14 +15,20 @@ namespace Novelly.Api.Tests;
 public class LoggingTests : ServiceTestFixture
 {
     [Test]
-    public void Fetching_a_missing_chapter_logs_a_warning_before_throwing()
+    public async Task Fetching_a_missing_chapter_returns_null_and_logs_at_information_not_warning()
     {
         var missingId = Guid.NewGuid();
 
-        Assert.That(() => Chapters.GetAsync(missingId), Throws.TypeOf<NotFoundException>());
+        var result = await Chapters.GetAsync(missingId);
 
-        var warning = ChapterLogs.Entries.Single(e => e.Level == LogLevel.Warning);
-        Assert.That(warning.Message, Does.Contain(missingId.ToString()));
+        Assert.Multiple(() =>
+        {
+            Assert.That(result, Is.Null);
+            Assert.That(ChapterLogs.Entries.Where(e => e.Level == LogLevel.Warning), Is.Empty);
+            Assert.That(
+                ChapterLogs.Entries,
+                Has.Some.Matches<CapturedLogEntry>(e => e.Level == LogLevel.Information && e.Message.Contains(missingId.ToString())));
+        });
     }
 
     [Test]