refactor: rename SelfTest/ to SelfTests/ for plan v4 consistency

Renames HellionChat/SelfTest/ to HellionChat/SelfTests/ (plural) to
match the folder convention used throughout the Build Suite Plan v4
Phase 6 file list. The singular name was introduced as a known
discrepancy in cb327b8 and is now resolved.

- git mv preserves full history via rename detection
- Namespace updated: HellionChat.SelfTest → HellionChat.SelfTests
- Plugin.cs qualifier updated: SelfTest. → SelfTests.
- Build: 0 errors, 0 warnings

HellionChat/Plugin.cs

@@ -456,7 +456,7 @@ public sealed class Plugin : IDalamudPlugin
             // SelfTest hooks live alongside the live registry — the steps
             // poll Active per frame and need the registry already wired.
             SelfTestRegistry.RegisterTestSteps([
-                new SelfTest.ThemeSwitchSelfTestStep(this),
+                new SelfTests.ThemeSwitchSelfTestStep(this),
             ]);
 
             // Plugin integrations register their IPC subscribers up-front so