test: dedupe the nested test doubles into one shared file

TestDbFactory was redeclared as a private nested class in 40 test files,
NullServiceProvider and StubNotesApi in 14 each, DefaultStub in 5 — 73
declarations, all semantically identical (StubNotesApi differed only in
formatting and type qualification). They now live in TestDoubles.cs next to the
existing StubWorkerClient, which was already the shared-double pattern in this
project.

tests/ClaudeDo.Ui.Tests/ViewModels/TasksIslandRegroupTests.cs

@@ -33,12 +33,6 @@ public class TasksIslandRegroupTests : IDisposable
         return new ClaudeDoDbContext(opts);
     }
 
-    private sealed class TestDbFactory : IDbContextFactory<ClaudeDoDbContext>
-    {
-        private readonly Func<ClaudeDoDbContext> _create;
-        public TestDbFactory(Func<ClaudeDoDbContext> create) => _create = create;
-        public ClaudeDoDbContext CreateDbContext() => _create();
-    }
 
     private TasksIslandViewModel BuildViewModel()
     {