Adopt defi-positions sim command for a new payload

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The current implementation is assuming outdated response payload

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PR Summary

Medium Risk
Changes the JSON schema used to unmarshal DeFi positions (renamed fields and nested token objects), so incorrect mapping would silently affect displayed balances/values; adds strong unit coverage to mitigate but existing integration tests may need updates for renamed JSON keys.

Overview
Updates sim evm defi-positions to match the latest defi-positions API response schema by renaming value fields (e.g. usd_value/total_usd_value -> value_usd/total_value_usd), adding chain, and switching from flat token metadata fields to nested token objects (token, underlying_token, token0, token1) including new fields like lending_pool, pool_id, salt, and NFT per-token details.

Adjusts positionDetails formatting to read symbols from the nested token structs via a new tokenSymbol helper, and adds a comprehensive unit test that unmarshals a real API JSON payload to ensure all supported position types (Erc4626/Tokenized/UniswapV2/Nft/NftV4) decode correctly, including null logos and aggregation breakdowns.

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