refactor(#28): IMP-28 L4 _parse_json dedup (4 modules -> src/json_utils)

Consolidate duplicate _parse_json helpers from content_editor.py /
design_director.py / kei_client.py (fuller form) and pipeline.py (simple form)
into shared src/json_utils.parse_json (strict superset). All 18 call-sites
preserved via `parse_json as _parse_json` alias import; no behavior change.

- src/json_utils.py (new): shared helper, fenced/plain-fence/bare-brace patterns
  + list-prefix cleanup fallback.
- tests/test_json_utils.py (new): 9 unit tests pinning parser semantics.
- src/content_editor.py / design_director.py: remove local helper +
  unused `import json` / `import re`.
- src/kei_client.py / pipeline.py: remove local helper; `json` / `re` retained
  (used elsewhere).

Targeted tests 9 passed; full pytest 374 passed (3 pre-existing scripts/
collection errors reproduce on baseline 909bf75, IMP-28 unrelated).

src/pipeline.py

@@ -36,6 +36,7 @@ from src.image_utils import get_image_sizes, embed_images
 from src.space_allocator import calculate_container_specs
 from src.slide_measurer import measure_rendered_heights, capture_slide_screenshot
 from src.config import settings
+from src.json_utils import parse_json as _parse_json
 
 logger = logging.getLogger(__name__)
 
@@ -2080,20 +2081,3 @@ def _convert_kei_judgment(
             new_adjs.append(adj)
 
     review_result["adjustments"] = new_adjs
-
-
-def _parse_json(text: str) -> dict[str, Any] | None:
-    """텍스트에서 JSON을 추출한다."""
-    patterns = [
-        r"```json\s*(.*?)```",
-        r"```\s*(.*?)```",
-        r"(\{.*\})",
-    ]
-    for pattern in patterns:
-        match = re.search(pattern, text, re.DOTALL)
-        if match:
-            try:
-                return json.loads(match.group(1).strip())
-            except json.JSONDecodeError:
-                continue
-    return None