fix: validate bounded JSONRPC frames and envelopes (Fixes #532)

Read complete bounded multi-header frames without losing byte boundaries, and reject invalid request envelopes before dispatch while preserving supported IDs and legacy method errors. Exercise exact limits, fragmentation, recovery and native wire compatibility.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: karthiknadig