fix(server): add error handling for malformed OTA manifest and missing sig file

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

server/test_ota_endpoint.py

@@ -68,3 +68,9 @@ def test_firmware_endpoint_missing_raises(tmp_path):
     import server.ota_endpoint as mod
     with pytest.raises(mod.FirmwareNotFoundError):
         ota_firmware_impl(firmware_dir=tmp_path)
+
+
+def test_check_malformed_manifest(tmp_path):
+    (tmp_path / "manifest.json").write_text("not valid json{{{")
+    result = ota_check_impl(current_version="1.0.0", firmware_dir=tmp_path)
+    assert result["update"] is False