Add tests and validation for EventTheme schema

Introduced extensive test cases to validate `EventTheme` schemas, ensuring proper handling of valid and invalid inputs, optional fields, and partial updates. Enhanced schema validation by adding constraints for `name`, `color_palette`, and `fonts` fields, as well as updating `id` to use `UUID` type for consistency. These changes improve data integrity and increase confidence in schema-related functionality.

backend/tests/schemas/test_event_theme_schema.py

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+import pytest
+from uuid import UUID
+from pydantic import ValidationError
+from app.schemas.event_theme import (
+    EventThemeCreate,
+    EventThemeUpdate,
+    EventThemeResponse,
+    EventTheme
+)
+
+
+class TestEventThemeSchemas:
+    def test_valid_theme_create(self, theme_data):
+        """Test creating a valid theme with all required fields"""
+        theme = EventThemeCreate(**theme_data)
+        assert theme.name == theme_data["name"]
+        assert theme.description == theme_data["description"]
+        assert theme.preview_image_url == theme_data["preview_image_url"]
+        assert theme.color_palette == theme_data["color_palette"]
+        assert theme.fonts == theme_data["fonts"]
+
+    @pytest.mark.parametrize("invalid_data,expected_error", [
+        (
+                {"name": "", "color_palette": {"primary": "#000"}, "fonts": {"main": "Arial"}},
+                "name"
+        ),
+        (
+                {"name": "Theme", "color_palette": {}, "fonts": {"main": "Arial"}},
+                "color_palette"
+        ),
+        (
+                {"name": "Theme", "color_palette": {"primary": "#000"}, "fonts": {}},
+                "fonts"
+        ),
+        (
+                {},
+                "name"
+        ),
+    ])
+    def test_invalid_theme_create(self, invalid_data, expected_error):
+        """Test validation errors for invalid theme data"""
+        with pytest.raises(ValidationError) as exc_info:
+            EventThemeCreate(**invalid_data)
+        assert expected_error in str(exc_info.value)
+
+    def test_theme_update_partial(self):
+        """Test updating theme with partial data"""
+        update_data = {
+            "name": "Updated Theme"
+        }
+        theme = EventThemeUpdate(**update_data)
+        assert theme.name == "Updated Theme"
+        assert theme.color_palette is None
+        assert theme.fonts is None
+
+    def test_theme_response(self, theme_data):
+        """Test theme response schema with ID"""
+        response_data = {
+            **theme_data,
+            "id": "123e4567-e89b-12d3-a456-426614174000"
+        }
+        theme = EventThemeResponse(**response_data)
+        assert isinstance(theme.id, UUID)
+        assert str(theme.id) == "123e4567-e89b-12d3-a456-426614174000"
+        assert theme.name == theme_data["name"]
+
+    def test_color_palette_validation(self):
+        """Test color palette validation"""
+        valid_data = {
+            "name": "Test Theme",
+            "color_palette": {"primary": "#000000", "secondary": "#FFFFFF"},
+            "fonts": {"main": "Arial"}
+        }
+        theme = EventThemeCreate(**valid_data)
+        assert theme.color_palette == valid_data["color_palette"]
+
+    def test_fonts_validation(self):
+        """Test fonts validation"""
+        valid_data = {
+            "name": "Test Theme",
+            "color_palette": {"primary": "#000000"},
+            "fonts": {"main": "Arial", "secondary": "Helvetica"}
+        }
+        theme = EventThemeCreate(**valid_data)
+        assert theme.fonts == valid_data["fonts"]
+
+    def test_optional_fields(self):
+        """Test optional fields handling"""
+        minimal_data = {
+            "name": "Minimal Theme",
+            "color_palette": {"primary": "#000000"},
+            "fonts": {"main": "Arial"}
+        }
+        theme = EventThemeCreate(**minimal_data)
+        assert theme.description is None
+        assert theme.preview_image_url is None
+
+    @pytest.mark.parametrize("field,value", [
+        ("name", "Updated Name"),
+        ("color_palette", {"new": "#123456"}),
+        ("fonts", {"new": "Times New Roman"}),
+        ("description", "New description"),
+        ("preview_image_url", "https://new-image.jpg"),
+    ])
+    def test_update_individual_fields(self, field, value):
+        """Test updating individual fields"""
+        update_data = {field: value}
+        theme = EventThemeUpdate(**update_data)
+        assert getattr(theme, field) == value
+
+    def test_theme_model_config(self):
+        """Test model configuration"""
+        assert hasattr(EventThemeResponse.Config, 'from_attributes')
+        assert EventThemeResponse.Config.from_attributes is True