Allow ReadAllLimit inputs at byte limit

[immanuwell]

Problem
ReadAllLimit was off by one: a body exactly at the configured cap returned limit reached. This can hit normal HTTP paths with 1MiB/10MiB/100MiB caps; no special infra magic needed, an exact-size request or response body does it.

Repro
Before this patch:

go test ./pkg/util -run TestReadAllLimit -count=1

fails on Allows input at limit with limit reached while reading: 5.

Fix
Read one byte past the cap, so exactly-cap bytes are ok and cap+1 still fails. tiny off-by-one cleanup, imo.

Validation

go test ./pkg/util -count=1
go test ./pkg/protohttp -count=1
go test ./controller/webhook -count=1
go test ./controller/heartbeat -count=1
go test ./pkg/version -count=1