fix(node_http2): validate serialized headers (#36041)
- protect the compact NUL-delimited JavaScript-to-Rust header
representation without changing Node's invalid-header behavior
- replace header-value code units that would become NUL after Latin-1
conversion with an invalid HTTP/2 byte for nghttp2 to reject or drop
- preserve only supported nghttp2 header flags in the native parser
- cover scalar, array, pseudo-header, non-ASCII low-byte, and native
flag handling
