Render a raw signed integer as signed

[moodysalem]

format_raw shares one match arm between ArgumentValue::Uint and
ArgumentValue::Int and puts both through BigUint::from_bytes_be, so every
negative value prints as its two's-complement word.

ArgumentValue::Uint(bytes) | ArgumentValue::Int(bytes) => {
    let n = BigUint::from_bytes_be(bytes);
    n.to_string()
}

A Uniswap-style tick of -140 renders as

115792089237316195423570985008687907853269984665640564039457584007913129639796

which is the part worth emphasising: it is not a number a reader can recognise
as wrong. It is a plausible-looking enormous integer, on the screen whose entire
job is telling someone what they are about to sign.

Why descriptors cannot work around it

ERC-7730 defines raw as "the natural representation of the underlying
structured data type", and for intN that is signed. The v2 schema has no
signed-integer format at all, so raw is the only thing a descriptor author
can reach for. A descriptor that wants a signed display has nowhere else to go.

In practice authors have been reaching for number, which renders correctly
here because format_number already calls int_to_bigint — but number is
not in the v2 schema, so those descriptors fail schema validation. Fixing the
engine lets them be spec-conformant and correct at the same time.

The change

Split the arm and point Int at the existing int_to_bigint, which already
does the two's-complement conversion for format_number:

ArgumentValue::Uint(bytes) => BigUint::from_bytes_be(bytes).to_string(),
ArgumentValue::Int(bytes) => int_to_bigint(bytes).to_string(),

The Uint arm is unchanged. The test pins both sides of the boundary — the
same 32-byte word read as Int and as Uint, plus zero, -1, and the sign-bit
edge — so the fix cannot later be simplified back into a shared arm.

format_raw_with_separator inherits it through its per-item format_raw call.
The EIP-712 renderer's Raw path is unaffected: it formats JSON values, where
a negative number already arrives as "-140" rather than as a word.

Testing

cargo test --workspace passes: 139 unit tests plus every integration suite,
with no existing expectation changed. Nothing in the corpus was relying on the
unsigned reading.

Found while tracking down why a wallet's approval screen showed a 78-digit
number for a liquidity position's lower tick.