test: add comprehensive integration tests for multi-field sorting (#373)

Added integration tests using TestContainers to verify multiple field
sorting behavior with real Redis Search instances. Tests cover:

- FilterQuery: single/multiple field sorting with various formats
- VectorQuery: custom sorting vs default vector distance sorting
- TextQuery: sorting text search results by custom fields
- Edge cases: invalid inputs, empty lists, None values
- Backward compatibility: existing sort_by usage patterns
- Warning verification: logs when multiple fields specified

All 17 integration tests validate that:
1. Single field sorting works in ASC/DESC modes
2. Multiple field specifications are accepted but only first field used
3. Warnings are logged when multiple fields provided
4. Filter expressions work correctly with sorting
5. Old-style API calls remain functional

Test data: 8 product records with varied price, rating, and stock values.

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: bsbodden