test(templates): cover instance_seed helper and update service tests

Add tests/service_testing/test_instance_seed.py — pins NODE_NAME to assert
determinism of seeded functions and sweeps NODE_NAMEs to assert cross-fleet
divergence. Conftest gains load_real_instance_seed() so template tests see
the real seeding behavior instead of a stub. Existing template tests updated
to pin NODE_NAME and match seeded outputs.

tests/service_testing/test_postgres.py

@@ -14,16 +14,21 @@ import pytest
 from hypothesis import given, settings
 from hypothesis import strategies as st
 
-from .conftest import _FUZZ_SETTINGS, make_fake_syslog_bridge, run_with_timeout
+from .conftest import (
+    _FUZZ_SETTINGS,
+    load_real_instance_seed,
+    make_fake_syslog_bridge,
+    run_with_timeout,
+)
 
 
 # ── Helpers ───────────────────────────────────────────────────────────────────
 
 def _load_postgres():
-    for key in list(sys.modules):
-        if key in ("postgres_server", "syslog_bridge"):
-            del sys.modules[key]
+    for key in ("postgres_server", "syslog_bridge", "instance_seed"):
+        sys.modules.pop(key, None)
     sys.modules["syslog_bridge"] = make_fake_syslog_bridge()
+    sys.modules["instance_seed"] = load_real_instance_seed()
     spec = importlib.util.spec_from_file_location("postgres_server", "decnet/templates/postgres/server.py")
     mod = importlib.util.module_from_spec(spec)
     spec.loader.exec_module(mod)