test: add characterization tests for core modules

33 tests covering env/io helpers, config parsers, pipeline meta I/O,
worker inbox logic, and FastAPI smoke routes. No gmic/rembg subprocesses.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_io_utils.py

@@ -0,0 +1,26 @@
+"""Tests for io_utils."""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+from app.io_utils import read_json, write_json_atomic
+
+
+def test_read_json_missing_returns_default(tmp_path: Path) -> None:
+    assert read_json(tmp_path / "missing.json", default={}) == {}
+
+
+def test_read_json_corrupt_returns_default(tmp_path: Path) -> None:
+    path = tmp_path / "bad.json"
+    path.write_text("not json", encoding="utf-8")
+    assert read_json(path, default=None) is None
+
+
+def test_write_json_atomic_roundtrip(tmp_path: Path) -> None:
+    path = tmp_path / "data.json"
+    payload = {"job_id": "photo", "variants": []}
+    write_json_atomic(path, payload)
+    assert json.loads(path.read_text(encoding="utf-8")) == payload
+    assert not path.with_suffix(path.suffix + ".tmp").exists()