graph: replace to_signed_u256 with fallible to_i256 (#6560)

to_signed_u256 panicked on values outside the int256 range, killing the
runner thread silently. The replacement to_i256 returns Result and is
plumbed through both reachable call sites (declared-call entity triggers
in data_source/common.rs, WASM ABI conversion in runtime/wasm) via the
existing Deterministic failure path.

to_unsigned_u256 had the symmetric latent panic for values >= 2^256
(via ruint's from_le_slice); add an explicit length check there too.

Removes the redundant U256-as-signed-transport API now that alloy
provides I256 directly.