feat(NODE-7097): use Node.js built-in zstd when available

[tadjik1]

Description

Summary of Changes

Use Node.js built-in zstd (zlib.zstdCompress / zlib.zstdDecompress, Node.js 22.15.0+) when available, falling back to @mongodb-js/zstd otherwise

Tests: interop with @mongodb-js/zstd in both directions, built-in preferred when both are present, fallback when built-in is unavailable

Release Highlight

zstd compression no longer requires @mongodb-js/zstd on Node.js 22.15.0+

The driver now uses Node.js' built-in zstd implementation when it is available. On older Node.js versions it continues to use @mongodb-js/zstd, so no changes are needed for existing setups.

Thanks to @axelonet for the original contribution!

Double check the following

• Lint is passing (npm run check:lint)
• Self-review completed using the steps outlined here
• PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• Changes are covered by tests
• New TODOs have a related JIRA ticket

[Copilot]

🟢 Approval recommended

The implementation preserves fallback behavior and includes focused coverage for the new selection logic.

0 open findings

What changed in this PR

Adds native Node.js zstd support while retaining the optional package fallback.

Changes:

• Prefer built-in zstd compression and decompression when available.
• Add interoperability, preference, compression-level, and fallback tests.
• Document when @mongodb-js/zstd is required.

File	Description
src/​deps.ts	Selects built-in zstd or the optional dependency.
test/​unit/​cmap/​wire_protocol/​compression.test.ts	Tests interoperability and implementation selection.
test/​mongodb_bundled.ts	Exposes the loader to bundled tests.
README.md	Updates zstd dependency guidance.

🧠 Review effort: Balanced

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[seanrmilligan]

By rewriting this test (and adding another way to do zstd compression) we might be changing the meaning of this string a little.

Should tests under this get a context of 'when the compression library is zlib'? And then, elsewhere, when the compression library is @mongodb-js/zstd?

[tadjik1]

Thanks @seanrmilligan! I renamed them to "when the agreed compressor is zstd" to match the snappy/zlib tests in the same file. The idea is that these tests pass whichever implementation the driver picks: on Node 22.15+ the interop tests check the built-in against @mongodb-js/zstd in both directions, and the separate getZstdLibrary() tests check which implementation is picked.