node:http2: reassemble HEADERS+CONTINUATION before HPACK decoding
The node:http2 frame parser decoded each HEADERS and CONTINUATION fragment independently and silently truncated the header list when an HPACK instruction straddled a fragment boundary, which also left the connection-scoped dynamic table out of sync. Fragments are now buffered per stream and decoded once when END_HEADERS arrives; a decode failure on a complete block tears down the connection with COMPRESSION_ERROR, any non-CONTINUATION frame interleaved into an unfinished header block is a PROTOCOL_ERROR, and the buffered compressed block is capped at maxHeaderListSize. The header list size/count limits now finish consuming the block before rejecting it so the dynamic table stays consistent.
request() now also spreads its options object before the header block is encoded (matching Node), so a user-supplied option getter that re-entrantly calls request() can no longer reorder header blocks on the wire relative to the order they were encoded.
Adds four raw-frame tests to test/js/node/http2/node-http2.test.js covering reassembly of a block split mid-instruction, decode failure of a complete block, an interleaved frame during reassembly, and an oversized compressed block. The existing CONTINUATION suite (150/300-header send and receive, large trailers, padding sweep) passes unchanged.