fix(explorers): return ParseOverflow instead of wrapping or panicking in parse_units - #283

parse_units scales the parsed amount by 10^(units - decimals), but the final multiplication was unchecked:

• on the unsigned path, U256 *= wraps silently. parse_units("115792089237316195423570985008687907853269984665640564039458", "ether") (one more than U256::MAX / 1e18) returned Ok(U256(415992086870360064)), which is about 0.4 ether instead of an error;
• on the negative path, I256 *= panics with overflow for the same amount with a leading -.

Both branches now use checked_mul and return the existing ConversionError::ParseOverflow, which the function already returns when 10^exponent itself overflows. Amounts that fit are unchanged. I also dropped the unused _fi local in the negative branch.

Tests. Added parse_units_rejects_overflow. It checks that the overflowing amount is rejected on both the positive and negative paths, and that 1.5 ether and the largest whole-ether amounts still parse. cargo test -p foundry-block-explorers --lib passes (31 tests), and cargo +nightly fmt and cargo clippy -p foundry-block-explorers --all-targets are clean.

This change was prepared with the help of an AI coding assistant.