fix: security hardening — amount validation, address validation, chain ID rejection

[livingproofdev25]

Summary

• Transfer amount validation — Added validateAmount() that rejects non-numeric, negative, zero, and excessively large amounts before parseEther()/parseUnits()/BigInt(). Applied to transferETH, transferERC20, approveERC20, and transferERC1155.
• Ethereum address validation — Added validateAddressOrEns() that validates 0x + 40 hex char format while allowing ENS names (containing .) to pass through. Applied to read_contract, write_contract, transfer_native, and transfer_erc20 tool handlers.
• Reject unknown chain IDs — resolveChainId() and getChain() now throw errors for unrecognized networks instead of silently falling back to Ethereum mainnet, preventing accidental transactions on the wrong chain.

Test plan

• Verify npx tsc --noEmit passes
• Verify transfer_native rejects negative amounts (e.g., "-1")
• Verify read_contract rejects malformed addresses (e.g., "0xinvalid")
• Verify ENS names like "vitalik.eth" still work
• Verify unknown network name (e.g., "typo") throws instead of defaulting to mainnet

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