CrewAI Agent and Task Definition

Define three CrewAI agents: 'Problem Decomposer' (primary model: Gemini 2.5 Pro), 'Knowledge Synthesizer' (primary model: Gemini 2.5 Pro), and 'Critical Evaluator' (primary model: GPT-4o). Assign specific, distinct roles and backstories. Create tasks for each: decomposition, research (using tools), synthesis, and critical review. Provide the initial Python code for defining these agents and tasks.