Refactor tests for improved readability and maintainability

- Consolidated duplicate fixture code into shared mcp_server fixture
- Extracted common test patterns into reusable helper functions:
  * generate_unique_index_name() for consistent naming
  * wait_for_indexing() for standardized wait times
  * create_test_index_with_documents() for test setup
  * assert_text_content_response() for common assertions
- Added configuration constants for test URLs and timing
- Simplified repetitive test patterns while preserving functionality
- Moved imports to top level for better organization
- Improved error messages and assertion clarity
- Reduced code duplication across test classes

All existing tests pass with identical functionality, now with better maintainability.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: tpayet