test: stub Range layout methods once in the test setup (#53)

Root cause: jsdom has no layout and CodeMirror measures text ranges in an animation frame, sometimes after a test finished; test files that did not stub Range.getClientRects (e.g. search/codePreview) threw an unhandled TypeError that failed whole CI runs at random (seen on #41).
Fix: stub Range.getClientRects / getBoundingClientRect once in src/test/setup.ts; drop the six per-file copies.
Verified: vitest on editor/lsp, editor/refactor, templates, search (55 files, 458 tests, no unhandled errors).

Claude-Session: https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

src/test/setup.ts

@@ -39,5 +39,9 @@ if (typeof window !== "undefined") {
       }) as MediaQueryList;
   }
   Element.prototype.scrollIntoView ??= function () {};
+  // No layout either: CodeMirror measures text ranges when it positions the caret,
+  // tooltips and line heights, sometimes in an animation frame after a test ends.
+  Range.prototype.getClientRects ??= () => Object.assign([], { item: () => null }) as unknown as DOMRectList;
+  Range.prototype.getBoundingClientRect ??= () => new DOMRect();
   Element.prototype.scrollTo ??= function () {} as Element["scrollTo"];
 }