fix: use quoted strings for size values in YAML to prevent time parsing

YAML parser interprets 16:9 as time format (16h 9m = 969s).
Using quoted strings "16:9" ensures correct string parsing.

tests/conftest.py

@@ -29,7 +29,7 @@ def project_root_dir():
 # ============= YAML 文件 Fixtures =============
 
 MINIMAL_YAML = """metadata:
-  size: 16:9
+  size: "16:9"
 
 slides:
   - background:
@@ -207,7 +207,7 @@ def yaml_with_all_elements(temp_dir):
     """创建包含所有元素类型的 YAML"""
     yaml_content = """
 metadata:
-  size: 16:9
+  size: "16:9"
 
 slides:
   - background:
@@ -292,7 +292,7 @@ def multiple_slides_yaml(temp_dir):
     """创建多张幻灯片的 YAML"""
     yaml_content = """
 metadata:
-  size: 16:9
+  size: "16:9"
 
 slides:
   # 第一张：标题页