json: report integer duration overflow against time.Duration

[sueun-dev]

decodeDuration parses integer durations with parseInt(b, durationType), but on error it returned inputError(b, int32Type) and then checked v < math.MinInt64 || v > math.MaxInt64. v is an int64, so that branch is unreachable (staticcheck SA4003, the lint reported in #150), and both error paths reported int32 instead of time.Duration.

Decoding an integer that overflows int64 into a time.Duration field showed this:

before: json: cannot unmarshal "100000000000000000000000}" into Go struct field ...D of type int32
after:  json: cannot unmarshal number 100000000000000000000000} overflows into Go struct field ...D of type time.Duration

time.Duration is backed by int64, so decodeDuration can propagate the parseInt error the same way decodeInt64 does, with no separate bounds check. Added a regression test; go test ./... passes and staticcheck no longer flags the line.

Fixes #150