Add failing E2E repro for #328: case-only rename is rejected as a conflict

Two tests fail on staging, proving the bug: `rpk servers rename Srv01 srv01`
hits the ConflictException at RenameResourceUseCase.cs:22 because
GetByNameAsync matches the resource against itself case-insensitively.
The third test passes and pins the behaviour that must survive the fix:
renaming onto a case variant of a *different* resource stays a conflict.

Agreed direction on the issue: allow a case-only rename as a re-casing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Tests/EndToEnd/RenameCasingTests.cs

@@ -0,0 +1,70 @@
+using Tests.EndToEnd.Infra;
+using Xunit.Abstractions;
+
+namespace Tests.EndToEnd;
+
+// Reproduces https://github.com/Timmoth/RackPeek/issues/328 (case D):
+// a case-only rename such as `rpk servers rename Srv01 srv01` is rejected with
+// "Conflict: Server resource 'srv01' already exists", because the existence
+// check matches the resource against itself. Agreed behaviour on the issue is
+// that a case-only rename should proceed as a re-casing, while renaming onto a
+// case variant of a *different* resource stays a conflict.
+[Collection("Yaml CLI tests")]
+public class RenameCasingTests(TempYamlCliFixture fs, ITestOutputHelper outputHelper)
+    : IClassFixture<TempYamlCliFixture> {
+    private async Task<(string output, string yaml)> ExecuteAsync(params string[] args) {
+        outputHelper.WriteLine($"rpk {string.Join(" ", args)}");
+
+        var output = await YamlCliTestHost.RunAsync(
+            args,
+            fs.Root,
+            outputHelper,
+            "config.yaml");
+
+        outputHelper.WriteLine(output);
+
+        var yaml = await File.ReadAllTextAsync(Path.Combine(fs.Root, "config.yaml"));
+        return (output, yaml);
+    }
+
+    [Fact]
+    public async Task case_only_rename_recases_the_resource() {
+        await File.WriteAllTextAsync(Path.Combine(fs.Root, "config.yaml"), "");
+
+        await ExecuteAsync("servers", "add", "Srv01");
+
+        (var output, var yaml) = await ExecuteAsync("servers", "rename", "Srv01", "srv01");
+
+        Assert.Equal("Server 'Srv01' renamed to 'srv01'.\n", output);
+        Assert.Contains("name: srv01", yaml);
+        Assert.DoesNotContain("name: Srv01", yaml);
+    }
+
+    [Fact]
+    public async Task case_only_rename_updates_dependants_and_connections() {
+        await File.WriteAllTextAsync(Path.Combine(fs.Root, "config.yaml"), "");
+
+        await ExecuteAsync("servers", "add", "Srv01");
+        await ExecuteAsync("systems", "add", "vm01", "--runs-on", "Srv01");
+
+        (var output, var yaml) = await ExecuteAsync("servers", "rename", "Srv01", "srv01");
+
+        Assert.Equal("Server 'Srv01' renamed to 'srv01'.\n", output);
+        Assert.Contains("- srv01", yaml);
+        Assert.DoesNotContain("- Srv01", yaml);
+    }
+
+    [Fact]
+    public async Task rename_onto_a_case_variant_of_another_resource_is_still_a_conflict() {
+        await File.WriteAllTextAsync(Path.Combine(fs.Root, "config.yaml"), "");
+
+        await ExecuteAsync("servers", "add", "Srv01");
+        await ExecuteAsync("servers", "add", "srv02");
+
+        (var output, var yaml) = await ExecuteAsync("servers", "rename", "srv02", "SRV01");
+
+        Assert.Contains("already exists", output, StringComparison.OrdinalIgnoreCase);
+        Assert.Contains("name: Srv01", yaml);
+        Assert.Contains("name: srv02", yaml);
+    }
+}