Fix node:zlib zstd "Unexpected end of file" on valid input

[t-kouyama]

Fixes #6769.

Zstd compression could throw "Unexpected end of file" for valid incompressible input because the sync encode path treated a non-zero return from ZSTD_compressStream2(..., ZSTD_e_end) as EOF even when the encoder still had output left to flush.

This change keeps the sync processing loop running while the zstd encoder still has pending input or output, while leaving the zlib and brotli sync paths unchanged. Since the callback-based API routes through the same sync implementation, this fixes both zstdCompressSync and zstdCompress.

The regression test adds a large random-input round-trip case to cover the same pending-output path.

Tests

• bazel test //src/workerd/api/node/tests:zlib-nodejs-test@
• bazel test //src/workerd/api/node/tests:zlib-zstd-nodejs-test@

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[t-kouyama]

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