fix: add regression test guarding devcontainer-lock.json JSON validity

tests/unit/test_devcontainer_json.py

@@ -0,0 +1,43 @@
+"""Regression test: .devcontainer JSON files must be valid JSON.
+
+Guard against a past failure where a stray closing brace was appended to
+devcontainer-lock.json, causing the devcontainers CLI to abort with:
+  SyntaxError: Unexpected non-whitespace character after JSON at position 2143
+
+See: GitHub Actions run 29139965422, job 86511118286.
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+DEVCONTAINER_DIR = REPO_ROOT / ".devcontainer"
+
+
+def test_devcontainer_json_is_valid():
+    path = DEVCONTAINER_DIR / "devcontainer.json"
+    assert path.exists(), f"{path} not found"
+    try:
+        json.loads(path.read_text(encoding="utf-8"))
+    except json.JSONDecodeError as exc:
+        raise AssertionError(f"devcontainer.json is invalid JSON: {exc}") from exc
+
+
+def test_devcontainer_lock_json_is_valid():
+    path = DEVCONTAINER_DIR / "devcontainer-lock.json"
+    assert path.exists(), f"{path} not found"
+    try:
+        json.loads(path.read_text(encoding="utf-8"))
+    except json.JSONDecodeError as exc:
+        raise AssertionError(
+            f"devcontainer-lock.json is invalid JSON: {exc}\n"
+            "Hint: check for a stray closing brace or bracket at the end of the file."
+        ) from exc
+
+
+def test_devcontainer_lock_json_has_features_key():
+    path = DEVCONTAINER_DIR / "devcontainer-lock.json"
+    data = json.loads(path.read_text(encoding="utf-8"))
+    assert "features" in data, "devcontainer-lock.json missing top-level 'features' key"