fix(e2e): reduce Hetzner batch parallelism from 3 to 2

Prevents server_limit_reached errors when pre-existing servers (e.g.
spawn-szil) consume quota during E2E batch 1.

Fixes #3111

Agent: test-engineer
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: louisgv

sh/e2e/lib/clouds/hetzner.sh

@@ -378,9 +378,9 @@ _hetzner_cleanup_stale() {
 # ---------------------------------------------------------------------------
 # _hetzner_max_parallel
 #
-# Hetzner accounts have a primary IP limit. This QA account supports ~3
-# concurrent provisioning operations before hitting resource_limit_exceeded.
+# Hetzner accounts have a primary IP limit. Reduced from 3 to 2 to avoid
+# server_limit_reached when pre-existing servers consume quota (#3111).
 # ---------------------------------------------------------------------------
 _hetzner_max_parallel() {
-  printf '3'
+  printf '2'
 }