fix: resolve 8 EVM handler bugs

[BitHighlander]

Summary

• net_version: stop double-converting decimal chainId as hex (BSC returned 86 instead of 56)
• convertToHex/handleTransfer: use ethers.parseEther instead of lossy parseFloat*1e18
• eth_gasPrice: handle null gasPrice on EIP-1559 chains
• wallet_addEthereumChain: require user approval before adding custom chains (was auto-approve)
• signTypedData: handle both JSON string and object params from dApps
• eth_sign: use correct EIP-191 param order [address, message]
• failedRpcs: TTL-based cleanup prevents unbounded Map growth

Test plan

• Switch to BSC, verify net_version returns "56" not "86"
• Send ETH transfer with many decimal places, verify precision
• Call eth_gasPrice on an EIP-1559 chain, verify no crash
• dApp calls wallet_addEthereumChain — verify approval popup shown
• Sign typed data from a dApp that sends object (not string)
• Test personal_sign and eth_sign both work correctly

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