test: replace dead duplicate signature test, drop ts-node, require node 22

[dantio]

Follow-up to #218, addressing review findings from the vitest migration.

Replace the dead duplicate signature test

should throw an error pseudo header is not known had a body byte-identical to the content-digest test above it — it asserted nothing new and never reached the branch its name described.

That branch is in fact unreachable: getSignatureParams only ever emits @method/@path/@authority, all of which the switch in generateBaseString handles, so default: throw new Error('Unknown pseudo header ...') (src/api/digitalSignature.ts:77) cannot be hit by any input. Rather than mock module internals to reach dead code, this replaces it with a case that is both reachable and genuinely untested: a missing x-ebay-signature-key header on the no-payload path. When payload is null, content-digest drops out of the signature params, so the key header is the first one checked.

Mutation-checked: altering the expected message fails the test, restoring it passes.

Drop ts-node

ts-node was only needed by .mocharc.json's loader, which #218 replaced with vitest. Removed from devDependencies, along with the now-orphaned "ts-node": {"esm": true} block in tsconfig.json.

The examples needed a replacement runner. Bare node does not work even on 22+ — native type stripping does not rewrite the .js import specifiers the examples use (import eBayApi from '../../../src/eBayApi.js'), so it fails with ERR_MODULE_NOT_FOUND. Swapped in tsx, which resolves them. Still dev-only. Also dropped the stale "Node 20" section from examples/README.md.

Bump engines.node to >= 22.0.0

Matches the CI matrix and the @types/node ^22 bump from #218. Note this is consumer-facing: npm will emit EBADENGINE for installs on Node < 22.

Test plan

Verified from a clean npm ci:

• npm test — 1371 passed, 9 files (unchanged count vs. master)
• npm run typecheck — pass
• npm run lint — pass
• npm ci — clean, 0 vulnerabilities
• No ts-node references remain outside the lockfile/CHANGELOG