Fetch DeFi yield pools from the yields.llama.fi API, optionally filtering by chain or project.
Returns symbol, project, tvlUsd, apy, apyMean30d, and predictions for each pool.
Args:
chain: Optional filter for blockchain (e.g., 'Ethereum', 'Solana')
project: Optional filter for project name (e.g., 'lido', 'aave-v3')