Refactor purple agent: Remove legacy code and make max_turns per-request

- Remove legacy src/agent.py (ShopperAgent with manual action parsing)
- Remove legacy src/llm_client.py (replaced by ADK's LiteLLM wrapper)
- Remove tests/test_agent.py (43 legacy tests)
- Clean up src/messenger.py: reduce from 383 to 35 lines
  - Keep only get_message_text() function (actively used)
  - Remove unused: create_message, merge_parts, Messenger class, send_message, parsing utilities
- Update src/shopping_agent.py: make max_turns configurable per request
  - Accept max_turns in task_data (defaults to instance default)
  - Pass to _execute_runner() as parameter
- Clean up legacy tests in test_executor.py and test_integration.py
- Add tests for per-request max_turns configuration

Impact:
- Removed ~2,180 lines of dead code
- All 103 tests passing
- Codebase focused on ADK-based architecture only

Author: mpnikhil