fix(toxics/bandwidth): don't hang or panic when rate <= 0

[jaideeppyne]

Problem

BandwidthToxic.Pipe already treats a non-positive rate as "no throttling" (if t.Rate <= 0 { sleep = 0 }), but the packet-splitting loop used t.Rate*100 as the split size unconditionally:

• rate == 0 → split size is 0, so len(p.Data) > 0 stays true forever; the loop spins emitting empty chunks (p.Data[:0]) and never forwards the payload — the connection hangs.
• rate < 0 → split size is negative; p.Data[:negative] panics with slice bounds out of range.

A bandwidth toxic created with rate: 0 (the zero value when the field is omitted) is enough to trigger the hang; there is no input validation rejecting rate <= 0.

Fix

Guard the split loop on t.Rate > 0 so a non-positive rate falls through to the normal unthrottled send path — matching the existing t.Rate <= 0 intent.

Tests

Added TestBandwidthToxicNonPositiveRate (rate_0, rate_-1), which fails before the change (empty forward / panic) and passes after. Full toxics package passes; gofmt/go vet clean.