[gracehopper-vt]

Summary

The VirusTotal API is adding a Retry-After header to rate-limit / quota-exceeded responses (429 in API v3). This PR exposes it to vt-go users:

• Error gets a new RetryAfter time.Duration field (json:"-"), set from the Retry-After response header whenever the API returns an error. It's zero when the header is absent or invalid.
• parseRetryAfter accepts both forms allowed by RFC 9110: delta-seconds (Retry-After: 120) and an HTTP date (http.ParseTime).
• Backwards compatible: existing fields and the Error() string are unchanged.

_, err := client.GetObject(vt.URL("files/%s", hash))
var vtErr vt.Error
if errors.As(err, &vtErr) && vtErr.Code == "QuotaExceededError" && vtErr.RetryAfter > 0 {
	time.Sleep(vtErr.RetryAfter)
}

No automatic retries are added: when a daily or monthly quota is exhausted, RetryAfter can be hours or days. Once this is released, vt-cli can use it to print a friendlier "retry in X" message.

Tests

• TestGetObjectOutOfQuotaRetryAfter: a 429 with Retry-After: 120 yields Error{Code: "QuotaExceededError", RetryAfter: 120s}.
• TestParseRetryAfter: empty, invalid, negative, seconds with whitespace, future HTTP date and past HTTP date.
• The test server gained SetResponseHeader.
• go test ./... passes. gofmt is clean for the touched files; the gofmt -l hits in feed.go, object.go and vt.go predate this change.