test(webauthn): select mock.module option shape by Node version (#692)

[millsmillsymills]

What

infra/lambdas/webauthn_demo/tests/verify-success.test.mjs mocked @simplewebauthn/server via mock.module(..., { namedExports }). The single exports option — and the DeprecationWarning for the legacy namedExports/defaultExport keys — landed in Node 25.9.0 (nodejs/node#61727, released 2026-04-01). CI pins Node 22 (only understands namedExports, silently ignores exports); local toolchains on Node 25.9+ emit the deprecation warning, tripping the zero-warnings policy on scripts/ci-local.sh.

This selects the option shape the running Node supports:

const [nodeMajor, nodeMinor] = process.versions.node.split('.').map(Number);
const supportsExportsOption = nodeMajor > 25 || (nodeMajor === 25 && nodeMinor >= 9);

• CI (Node 22) → namedExports branch — byte-equivalent to the prior code, stays green.
• Local (Node 25.9+) → exports branch — warning-free.

A naive swap to exports was rejected: it silently no-ops the mock on Node 22 (unknown option ignored → verified:true never forced → the four success tests fail). Verified empirically against the real mock.module API on Node 22 vs 25.9.

Verification

Exact CI command on local Node 25.9.0:

node --experimental-test-module-mocks --disable-warning=ExperimentalWarning --test 'infra/lambdas/webauthn_demo/tests/**/*.test.mjs'
→ tests 41 / pass 41 / fail 0 — NO warnings (previously: DeprecationWarning on namedExports)

Closes #692

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