Use block-scoped namespaces in the test suite

Converts all 73 test source files from file-scoped (namespace X;) to
block-scoped (namespace X { ... }) declarations, re-indenting their
contents accordingly. Matches the existing convention already used in
several production files (e.g. Musify.Infrastructure.Configuration,
Musify.Application.Contracts).

Mechanical change only, done with a script, verified with a full
rebuild (0 warnings, 0 errors) and test run (257/257 passing).

backend/Tests/Musify.Application.Tests/Users/GetUserByIdQueryHandlerTests.cs

@@ -3,29 +3,30 @@ using Musify.Application.Tests.TestSupport;
 using Musify.Application.Users;
 using Xunit;
 
-namespace Musify.Application.Tests.Users;
-
-public sealed class GetUserByIdQueryHandlerTests : HandlerTestBase
+namespace Musify.Application.Tests.Users
 {
-    private GetUserByIdQueryHandler CreateHandler() => new(Database);
-
-    [Fact]
-    public async Task Handle_ExistingUser_ReturnsIt()
+    public sealed class GetUserByIdQueryHandlerTests : HandlerTestBase
     {
-        var user = TestEntities.User(1, "Jane Doe");
-        await SeedAsync(user);
+        private GetUserByIdQueryHandler CreateHandler() => new(Database);
 
-        var result = await CreateHandler().Handle(new GetUserByIdQuery(1), TestContext.Current.CancellationToken);
+        [Fact]
+        public async Task Handle_ExistingUser_ReturnsIt()
+        {
+            var user = TestEntities.User(1, "Jane Doe");
+            await SeedAsync(user);
 
-        Assert.False(result.IsError);
-        Assert.Equal("Jane Doe", result.Value.Name);
-    }
+            var result = await CreateHandler().Handle(new GetUserByIdQuery(1), TestContext.Current.CancellationToken);
 
-    [Fact]
-    public async Task Handle_UserMissing_ReturnsNotFound()
-    {
-        var result = await CreateHandler().Handle(new GetUserByIdQuery(404), TestContext.Current.CancellationToken);
+            Assert.False(result.IsError);
+            Assert.Equal("Jane Doe", result.Value.Name);
+        }
 
-        Assert.Equal(ErrorType.NotFound, result.FirstError.Type);
+        [Fact]
+        public async Task Handle_UserMissing_ReturnsNotFound()
+        {
+            var result = await CreateHandler().Handle(new GetUserByIdQuery(404), TestContext.Current.CancellationToken);
+
+            Assert.Equal(ErrorType.NotFound, result.FirstError.Type);
+        }
     }
 }