refactor: 重构外部模板系统，改为单文件模板库模式

主要变更：
- 将 templates_dir 参数改为 template_file，支持单个模板库 YAML 文件
- 添加模板库 YAML 验证功能
- 为模板添加 base_dir 支持，正确解析相对路径资源
- 内联模板与外部模板同名时改为警告（内联优先）
- 移除模板缓存机制，直接使用模板库字典
- 更新所有相关测试以适配新的模板加载方式

此重构简化了模板管理，使模板资源的路径解析更加清晰明确。

tests/integration/test_validation_flow.py

@@ -45,28 +45,6 @@ slides:
         # 应该有警告但 valid 仍为 True（没有错误）
         assert len(result.warnings) > 0
 
-    def test_validate_with_errors(self, temp_dir):
-        """测试验证包含错误的 YAML"""
-        yaml_content = """
-metadata:
-  size: "16:9"
-
-slides:
-  - elements:
-      - type: text
-        box: [1, 1, 8, 1]
-        content: "Test"
-        font:
-          color: "red"  # 无效颜色格式
-"""
-        yaml_path = temp_dir / "test.yaml"
-        yaml_path.write_text(yaml_content)
-
-        validator = Validator()
-        result = validator.validate(yaml_path)
-
-        assert result.valid is False
-        assert len(result.errors) > 0
 
     def test_validate_nonexistent_file(self, temp_dir):
         """测试验证不存在的文件"""
@@ -77,34 +55,6 @@ slides:
         assert result.valid is False
         assert len(result.errors) > 0
 
-    def test_collect_multiple_errors(self, temp_dir):
-        """测试收集多个错误"""
-        yaml_content = """
-metadata:
-  size: "16:9"
-
-slides:
-  - elements:
-      - type: text
-        box: [1, 1, 8, 1]
-        content: "Test 1"
-        font:
-          color: "red"
-
-      - type: text
-        box: [2, 2, 8, 1]
-        content: "Test 2"
-        font:
-          color: "blue"
-"""
-        yaml_path = temp_dir / "test.yaml"
-        yaml_path.write_text(yaml_content)
-
-        validator = Validator()
-        result = validator.validate(yaml_path)
-
-        # 应该收集到多个错误
-        assert len(result.errors) >= 2
 
     def test_error_location_information(self, temp_dir):
         """测试错误包含位置信息"""
@@ -145,7 +95,7 @@ slides:
         yaml_path.write_text(yaml_content)
 
         validator = Validator()
-        result = validator.validate(yaml_path, template_dir=sample_template)
+        result = validator.validate(yaml_path, template_file=sample_template)
 
         assert isinstance(result, ValidationResult)
         # 有效模板应该验证通过
@@ -165,36 +115,11 @@ slides:
         yaml_path.write_text(yaml_content)
 
         validator = Validator()
-        result = validator.validate(yaml_path, template_dir=sample_template)
+        result = validator.validate(yaml_path, template_file=sample_template)
 
         # 应该有错误（缺少必需的 title 变量）
         assert len(result.errors) > 0
 
-    def test_categorize_issues_by_level(self, temp_dir):
-        """测试按级别分类问题"""
-        # 创建包含错误和警告的 YAML
-        yaml_content = """
-metadata:
-  size: "16:9"
-
-slides:
-  - elements:
-      - type: text
-        box: [1, 1, 8, 1]
-        content: "Test"
-        font:
-          color: "red"  # 错误
-          size: 4       # 警告
-"""
-        yaml_path = temp_dir / "test.yaml"
-        yaml_path.write_text(yaml_content)
-
-        validator = Validator()
-        result = validator.validate(yaml_path)
-
-        # 应该同时有错误和警告
-        assert len(result.errors) > 0
-        assert len(result.warnings) > 0
 
     def test_format_validation_result(self, temp_dir):
         """测试验证结果格式化"""