format: reject signed numeric tokens in ipv4 and time

[sueun-dev]

The ipv4 and time format validators parse their numeric tokens with strconv.Atoi, which accepts a leading +/- sign, so signed tokens validate even though the dotted-quad and RFC 3339 grammars are digit-only:

ipv4       "1.2.3.+4"              -> valid (want invalid)
ipv4       "+1.2.3.4"              -> valid
ipv4       "1.2.3.-0"              -> valid
time       "+9:30:00Z"            -> valid
time       "12:+9:00Z"            -> valid
time       "12:00:00+09:+0"       -> valid
date-time  "2020-01-01T+9:30:00Z" -> valid  (date-time reuses the time check)
email      "a@[1.2.3.+4]"         -> valid  (bracketed IPv4 literal reuses ipv4)

validateDuration and validateSemver in the same file already reject non-digit characters with an explicit loop. This adds a small isAllDigits helper and applies it before the three strconv.Atoi call sites (ipv4 octet, time hh:mm:ss, offset hh:mm), so date-time and email inherit the fix.

Added TestFormatRejectsSignedNumericTokens. Checked: go test ./... (including the official JSON-Schema-Test-Suite), go vet ./..., gofmt -l.