fix(ext/node): avoid trailing empty zstd frames

[ArakawaHenri]

Fixes #36688

ZstdCompress previously implemented an end write as run() followed by finish(). When the output buffer filled, the retry could insert ZSTD_e_continue into an in-progress ZSTD_e_end operation and append an empty frame.

This change:

• calls ZSTD_compressStream2() once per write, preserving the requested continue, flush, or end directive across output chunks;
• records when ZSTD_e_end completes while exactly filling the output buffer, preventing the following empty-input retry from starting another frame;
• shares the corrected path between synchronous and asynchronous writes;
• adds regressions for ordinary multi-chunk output and exact-fill completion.

Node parity

Node v26.7.0 passes its stored flush directive directly to ZSTD_compressStream2(). Matching that behavior restores Node parity for ordinary multi-chunk output.

Node's processChunk loop retries whenever availOutAfter === 0, so Node v26.7.0 currently appends an empty frame in the exact-fill case as well. This PR intentionally does not reproduce that boundary artifact: it depends only on internal output-buffer allocation and breaks consumers that validate exactly one frame. The return value from ZSTD_compressStream2() allows Deno to distinguish completed output from pending output.

The parity fix and the exact-fill guard are split into separate commits. If strict parity with Node's current exact-fill behavior is preferred, the first commit can be taken on its own.

Tests

• ./tools/format.js --check
• cargo check -p deno_node --features deno_core/v8
• ./x test-node zstd
• ./tools/lint.js

AI disclosure: ChatGPT assisted with bug investigation and patch implementation. I reviewed the changes and test results.