test(uitests): write failure diagnostics to a file, not print()

Confirmed (pasted full CI log around a timeout, searched for the
print()'d marker string, found nothing) that plain print() from inside
a UI test never reaches fastlane's xcodebuild log output on this
runner - its formatter only relays lines matching its own known
patterns and drops everything else.

CartCheckoutFlowTests now writes app.debugDescription to
NSTemporaryDirectory()+'pedifoods_uitest_diag.log' on each of its three
failure points (shared dumpDiagnostics helper, appending), clearing any
stale copy in class setUp(). test.yml gained a step after 'Run tests
with coverage' (if: always()) that cats that file when present - TMPDIR
is stable for the whole CI job, unlike the per-run-hashed workspace
path that broke the original screenshot attempt.

See decisions/2026-09-11-ui-test-shared-login-session.md follow-up.

.gitea/workflows/test.yml

@@ -24,3 +24,12 @@ jobs:
 
       - name: Run tests with coverage
         run: fastlane tests
+
+      - name: Show UI test failure diagnostics (if any)
+        if: always()
+        run: |
+          if [ -f "$TMPDIR/pedifoods_uitest_diag.log" ]; then
+            cat "$TMPDIR/pedifoods_uitest_diag.log"
+          else
+            echo "No UI test diagnostic log was written."
+          fi