test(qoder): add regression coverage for review-fix changes

Adds 18 new tests covering the bugs fixed in the previous commit so they
can't silently regress:

- parseExpiry (7 tests): numeric ms-epoch input, numeric strings handled
  before Date.parse so "1700000000" doesn't get year-interpreted, RFC3339
  strings, expires_in:0 honored as already-expired, 30-day fallback only
  when both inputs are missing/invalid

- normalizeMessages (4 tests): system hoisting, multipart text flatten,
  multiple system joining, empty input

- wrapQoderSSE (6 tests): the fixed cases — trailing partial line drained
  in flush(), no chunks forwarded after [DONE], embedded newlines stripped
  from inner body, error envelope produces error chunk + [DONE], non-ok
  responses returned unchanged

- expose parseExpiry from auth.js, expose normalizeMessages/wrapQoderSSE
  via __test__ from the executor (internals only — not part of the public
  API). Marked with comment so the surface is intentional.

42 tests total (24 original + 18 new). Build still clean.

src/lib/qoder/auth.js

@@ -186,7 +186,7 @@ export async function fetchUserInfo(accessToken) {
  * otherwise return a misleading year-2026 timestamp instead of falling
  * through to the integer branch.
  */
-function parseExpiry(expiresAt, expiresInSeconds) {
+export function parseExpiry(expiresAt, expiresInSeconds) {
   if (typeof expiresAt === "number" && Number.isFinite(expiresAt) && expiresAt > 0) {
     return expiresAt;
   }