improvement: pin Node.js version via .node-version and engines field

[trishtzy]

Align local dev and CI on a single Node.js version pinned in .node-version, so contributors using
fnm/nvm/asdf/nodenv/mise pick up a compatible runtime automatically and CI no longer drifts
from the version contributors run locally. Matches the Node 20 runtime already used by the Dockerfile
(node:20-alpine).

Description: (required)

• Add .node-version (20.20.2, the latest 20.x LTS patch at time of writing). Consumable by nvm,
  fnm, asdf-nodejs, nodenv, and mise. Uses a full MAJOR.MINOR.PATCH so it resolves
  out-of-the-box across all listed managers (nodenv and asdf-nodejs do not resolve partial versions by
  default — see Allow setting version by major version number only nodenv/nodenv#240 / Support x.x format version strings nodenv/nodenv#53).
• Add "engines": { "node": ">=20.9.0" } to package.json so npm install surfaces a warning on
  mismatch. 20.9.0 is the first active-LTS release of the Node 20 line — same idiom as Next.js
  (>=20.9.0) and Hono (>=16.9.0). The upper bound is intentionally left open; the Node 24 crash
  tracked in fix: Node.js v24 crash — 'Failed to find Response internal state key' #1604 is a dependency bug that should be fixed in @hono/node-server, not papered over here.
• Update .github/workflows/check_code_formatting.yml and .github/workflows/npm_publish.yml to read
  node-version-file: '.node-version' instead of hardcoding node-version: '20.x', giving the repo a
  single source of truth for Node across local dev and CI. Bumps actions/setup-node from v2 → v4 and
  v3 → v4 respectively (required: node-version-file was added in v3).
• Document the pin in docs/installation-deployments.md under the NodeJS Server section so new
  contributors know where the version comes from.

Tests Run/Test cases added: (required)

• No runtime code changes — no unit/integration tests added.
• Verified .node-version satisfies the engines.node range locally (20.20.2 ≥ 20.9.0).
• Verified actions/setup-node@v4 supports node-version-file per upstream
  docs.
• Ran npm run format:check — passes on package.json (repo's format glob excludes .md /
  .node-version / .yml, so no Prettier impact there).
• CI will exercise the two updated workflows on this PR — confirming Check Prettier Formatting
  runs green against the pinned version.

Type of Change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Note on "Breaking change": adding engines.node is technically observable for consumers of the
@portkey-ai/gateway npm package who have engine-strict=true in their .npmrc and run Node
<20.9.0. Given the gateway is deployed as a server (not imported as a library) and both the
Dockerfile and CI already assume Node 20, the practical impact is zero. Flagging for transparency —
happy to re-categorize if maintainers prefer.