fix(harness): preserve session migration JSON errors

.superpowers/sdd/task-2-report.md

@@ -149,3 +149,29 @@ session while the adapter executes as `thoth_sessions_runtime`.
 - Task 7 must safely provision the dedicated runtime login/membership and inject its TLS
   credentials/CA; this task intentionally does not create a deployment-specific LOGIN role or
   password.
+
+## Review follow-up — unavailable migration database JSON contract
+
+### RED
+
+Command:
+
+```sh
+cd harness && .venv/bin/pytest \
+  tests/test_session_migrate_cmd.py::test_session_migrate_status_database_failure_is_pristine_json -q
+```
+
+Result: `1 failed in 0.46s`. The unreachable direct PostgreSQL URL exited with code 1 but left
+stdout empty, so `json.loads(result.stdout)` raised `JSONDecodeError`.
+
+### GREEN
+
+The session migration CLI now catches `SQLAlchemyError` at the same command boundary as its
+migration/domain errors and emits only `{"error": ...}` on stdout for `--json`.
+
+```sh
+cd harness && .venv/bin/pytest tests/test_session_migrate_cmd.py -q
+cd harness && .venv/bin/ruff check tht/cli/session_cmd.py tests/test_session_migrate_cmd.py
+```
+
+Result: `2 passed in 3.60s`; Ruff: `All checks passed!`.

harness/tests/test_session_migrate_cmd.py

@@ -26,3 +26,22 @@ def test_session_migrate_status_is_pristine_and_idempotent():
     assert second.exit_code == 0, second.output
     assert json.loads(first.stdout) == expected
     assert json.loads(second.stdout) == expected
+
+
+def test_session_migrate_status_database_failure_is_pristine_json():
+    result = CliRunner().invoke(
+        app,
+        [
+            "session",
+            "migrate",
+            "--database-url",
+            "postgresql+psycopg2://test:test@127.0.0.1:1/test",
+            "--status",
+            "--json",
+        ],
+    )
+
+    assert result.exit_code == 1
+    assert result.stderr == ""
+    assert "Traceback" not in result.stdout
+    assert json.loads(result.stdout)["error"]

harness/tht/cli/session_cmd.py

@@ -2,6 +2,7 @@ import json
 from pathlib import Path
 
 import typer
+from sqlalchemy.exc import SQLAlchemyError
 
 from tht.cli.config_cmd import CONFIG_OPT
 from tht.cli.schema_cmd import _load_config_or_exit
@@ -22,7 +23,7 @@ def migrate_cmd(
 
     try:
         status = migration_status(database_url) if status_only else migrate(database_url)
-    except (MigrationError, ValueError) as exc:
+    except (MigrationError, SQLAlchemyError, ValueError) as exc:
         if json_output:
             typer.echo(json.dumps({"error": str(exc)}, sort_keys=True))
         else: