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/ExceptionHandlingTests.cs

@@ -0,0 +1,57 @@
+using Novelly.Api.Chapters;
+using Novelly.Api.Common;
+using Novelly.Api.Common.Validation;
+using Novelly.Api.Projects;
+
+namespace Novelly.Api.Tests;
+
+/// <summary>
+/// Covers the exception-handling rework: a missing entity is an ordinary result, not a
+/// thrown exception; <see cref="Guard"/> rejects missing required arguments; and a
+/// service re-validates a request even when a direct caller skips the API's own filter.
+/// </summary>
+[TestFixture]
+public class ExceptionHandlingTests : ServiceTestFixture
+{
+    [Test]
+    public void Guard_rejects_an_empty_guid_passed_as_a_required_id() =>
+        Assert.That(() => Projects.GetAsync(Guid.Empty), Throws.TypeOf<ArgumentException>());
+
+    [Test]
+    public void Guard_rejects_a_null_request_object() =>
+        Assert.That(
+            () => Projects.CreateAsync(null!),
+            Throws.TypeOf<ArgumentNullException>());
+
+    [Test]
+    public async Task Deleting_a_missing_project_returns_false_rather_than_throwing() =>
+        Assert.That(await Projects.DeleteAsync(Guid.NewGuid()), Is.False);
+
+    [Test]
+    public void A_blank_title_fails_the_create_project_validator()
+    {
+        var result = new CreateProjectRequestValidator().Validate(new CreateProjectRequest(""));
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(result.IsInvalid, Is.True);
+            Assert.That(result.Errors.Select(e => e.PropertyName), Has.Member("Title"));
+        });
+    }
+
+    [Test]
+    public void Calling_a_service_directly_with_an_invalid_request_throws_rather_than_silently_accepting_it() =>
+        Assert.That(
+            () => Projects.CreateAsync(new CreateProjectRequest("")),
+            Throws.TypeOf<ArgumentException>());
+
+    [Test]
+    public async Task An_embedded_reference_to_a_missing_parent_still_throws()
+    {
+        // Creating a chapter under a nonexistent project isn't a "look this up" miss — it's
+        // an invalid precondition for the create, so it stays exceptional.
+        Assert.That(
+            async () => await Chapters.CreateAsync(Guid.NewGuid(), new CreateChapterRequest("Landfall")),
+            Throws.TypeOf<NotFoundException>());
+    }
+}