Reject oversized positive and extended clipboard lengths at the header. Process coalesced WebSocket data in bounded slices without rejecting valid multi-message frames. Includes fragmented-header and boundary regressions. Addresses the clipboard finding reported by carlotestor and corroborated by helix4u and other reviewers on #108914. (cherry picked from commit b1fbe363266e6d6fa3d73d2605fe1ca383607859)