feat: sync research and tasks endpoints

[Shedna]

Summary

• add the public research lifecycle to the Python SDK (research, async_research, list_research, get_research, create_tasks, list_tasks, get_task, and async counterparts)
• add typed task and pagination models for the new public endpoints
• update tests and README examples for the new public surface

Drift Addressed

• linkup-platform exposes public research and tasks endpoints that were missing from the Python SDK
• the Python SDK now keeps the new task and research responses snake_case on the public surface while mapping to API wire format at the boundary

Validation

• uv run pytest
• uv run --with ruff ruff check .
• uv run mypy src