fix: keep the default response encoding stable across Node.js versions

[rxits]

Fixes #369

Regression

Since v8.1.0, the default response compression changes based on the Node.js version: under Node.js < 22.15 the default is br, but under Node.js ≥ 22.15 (where zlib.createZstdCompress exists) the same app defaults to zstd. Upgrading Node silently changes the wire format, which breaks consumers that don't expect/recognize zstd.

Cause

processCompressParams / processDecompressParams build the supported-encodings list and then unshift('zstd') to the front whenever the runtime supports it:

const supportedEncodings = ['br', 'gzip', 'deflate', 'identity']
if (typeof zlib.createZstdCompress === 'function') {
  supportedEncodings.unshift('zstd')   // zstd becomes the top server preference
}

@fastify/accept-negotiator uses this order to break ties, so a normal client sending Accept-Encoding: gzip, deflate, br, zstd now negotiates zstd instead of br.

Fix

Insert zstd before identity rather than at the front, so br stays the negotiated default (matching pre-8.1.0 behavior) while zstd is still offered when a client explicitly prefers it. identity remains last. Applied to both the compress and decompress encoding lists.

Tests

Added a test asserting that a client accepting gzip, deflate, br, zstd gets br (not zstd) on a zstd-capable runtime. Verified failing before the fix (expected 'br', actual 'zstd') and passing after. Full unit suite: 174/174 pass, lint clean. The existing explicit-zstd and wildcard (* → gzip) tests are unaffected.