feat(agent): add durable existing-agent credential transitions

backend/tests/test_legacy_agent_migrations.py

@@ -0,0 +1,75 @@
+"""Focused source-level contracts for legacy Agent migrations."""
+
+import importlib.util
+import unittest
+from pathlib import Path
+from unittest.mock import patch
+
+
+VERSIONS = Path(__file__).parents[1] / "alembic" / "versions"
+
+
+def load_migration(filename):
+    path = VERSIONS / filename
+    spec = importlib.util.spec_from_file_location(filename.removesuffix(".py"), path)
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+class EmptyResult:
+    def all(self):
+        return []
+
+
+class RecordingConnection:
+    def __init__(self):
+        self.statements = []
+
+    def execute(self, statement, parameters=None):
+        del parameters
+        self.statements.append(str(statement))
+        return EmptyResult()
+
+
+class LegacyRoutingMigrationTests(unittest.TestCase):
+    def test_0018_locks_every_preflight_and_update_table_before_reads(self):
+        migration = load_migration("0018_legacy_agent_routing.py")
+        connection = RecordingConnection()
+        with patch.object(migration.op, "get_bind", return_value=connection):
+            migration.upgrade()
+
+        first = " ".join(connection.statements[0].split()).lower()
+        self.assertTrue(first.startswith("lock table"))
+        self.assertIn("in share row exclusive mode", first)
+        for table in (
+            "agent_zones", "minerops_agent_discoveries",
+            "minerops_agent_snapshots", "minerops_site_agents", "sites",
+        ):
+            with self.subTest(table=table):
+                self.assertIn(table, first)
+
+        first_update = next(
+            index for index, statement in enumerate(connection.statements)
+            if "UPDATE " in statement
+        )
+        self.assertEqual(first_update, 4)
+
+    def test_0019_is_the_direct_forward_only_successor(self):
+        migration = load_migration("0019_agent_legacy_auth_retire.py")
+        self.assertEqual(migration.revision, "0019_agent_legacy_auth_retire")
+        self.assertEqual(migration.down_revision, "0018_legacy_agent_routing")
+
+    def test_0020_adds_only_existing_identity_transition_binding(self):
+        migration = load_migration(
+            "0020_existing_agent_credential_transition.py"
+        )
+        self.assertEqual(migration.revision, "0020_existing_agent_transition")
+        self.assertEqual(
+            migration.down_revision,
+            "0019_agent_legacy_auth_retire",
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()