refactor: extract setup factories from TaskManager

[san0808]

Summary

• Moves 7 setup/init methods from TaskManager into standalone functions in setup_factories.py
• Adds TaskType enum to bolna/enums.py, replaces all raw string comparisons
• DRYs repeated kwargs injection in get_agent_object with _inject_kwargs_into_config helper
• Converts __is_multiagent, __is_knowledgebase_agent, __is_graph_agent from double-underscore (name-mangled) to single-underscore (accessible from extracted code)
• Thin delegation stubs on TaskManager preserve internal call-sites