fix: treat negative LimitRule.RandomDelay as no delay

[Noethix55555]

Fixes #910

Problem

LimitRule.RandomDelay is an exported time.Duration that users set directly. The delay path in http_backend.go feeds it to rand.Int63n:

if r.RandomDelay != 0 {
    randomDelay = time.Duration(rand.Int63n(int64(r.RandomDelay)))
}

rand.Int63n panics for any argument <= 0, so a negative RandomDelay panics with invalid argument to Int63n. In Async mode the panic happens inside the fetch goroutine and crashes the whole process, so Wait() never returns.

Fix

Change the guard from != 0 to > 0 so a negative RandomDelay is treated as no random delay. This mirrors the fact that a negative Delay is already a harmless no-op for time.Sleep.

Test

Added TestLimitRuleNegativeRandomDelay, which configures a rule with RandomDelay: -1 * time.Second, performs an async Visit against an httptest server, and asserts Wait() completes without panicking. It fails (panics) on the current code and passes with the fix. Full go test ./... passes.