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/TestDoubles.cs

@@ -0,0 +1,32 @@
+using ClaudeDo.Data;
+using ClaudeDo.Ui.Services;
+using ClaudeDo.Ui.Services.Interfaces;
+using Microsoft.EntityFrameworkCore;
+
+namespace ClaudeDo.Ui.Tests;
+
+/// <summary>
+/// The test doubles every ViewModel test needs. They used to be redeclared as a private nested
+/// class in each test file (TestDbFactory alone in 40 of them) — put new shared no-op doubles here
+/// rather than copying one again. Test-specific behaviour still belongs in the test file.
+/// </summary>
+public sealed class TestDbFactory(Func<ClaudeDoDbContext> create) : IDbContextFactory<ClaudeDoDbContext>
+{
+    public ClaudeDoDbContext CreateDbContext() => create();
+}
+
+public sealed class NullServiceProvider : IServiceProvider
+{
+    public object? GetService(Type serviceType) => null;
+}
+
+public sealed class StubNotesApi : INotesApi
+{
+    public Task<List<DailyNoteDto>> ListAsync(DateOnly day) => Task.FromResult(new List<DailyNoteDto>());
+    public Task<DailyNoteDto?> AddAsync(DateOnly day, string text) => Task.FromResult<DailyNoteDto?>(null);
+    public Task UpdateAsync(string id, string text) => Task.CompletedTask;
+    public Task DeleteAsync(string id) => Task.CompletedTask;
+}
+
+/// <summary><see cref="StubWorkerClient"/> is abstract — this is the concrete no-op subclass.</summary>
+public sealed class DefaultStub : StubWorkerClient { }