doc(website): add an ADR to standardize json messages testing

docs/website/adr/008-standardize-json-messages-testing.md

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+---
+slug: 8
+title: |
+  8. Standardize JSON Message Testing
+authors:
+  - name: Mithril Team
+tags: [Accepted]
+date: 2025-01-14
+---
+
+## Status
+
+Accepted
+
+## Context
+
+- To ensure backward compatibility and correctness of JSON messages exchanged between nodes, we need a standardized approach
+to test the deserialization of these messages.
+- Golden testing is a technique where the expected output (golden data) is stored and used to verify the correctness of
+  the system's output. This approach helps in detecting unintended changes in the output and ensures that the system
+  behaves as expected over time.
+- By using golden testing for JSON message deserialization, we can ensure that any changes to the message structures are
+  backward compatible and that the deserialization process yields the expected results.
+- We have been using golden testing for JSON messages in the project, but the approach used ad-hoc versions that did not
+  correspond to any OpenAPI versions, making it difficult to track the changes and maintain backward compatibility.
+
+## Decision
+
+We will standardize the testing of JSON messages by following the steps below:
+
+When adding a new JSON message structure, the following steps should be taken:
+- Introduce a constant `CURRENT_JSON` string containing an exhaustive example of the JSON currently exchanged between nodes.
+- Implement a `golden_message_current` method that returns the representation of the `CURRENT_JSON` using the current structure.
+- Implement a `test_current_json_deserialized_into_current_message` test that checks that deserializing the `CURRENT_JSON` into the current structure yields the output stored in `golden_message_current`.
+
+When modifying an existing JSON message structure, if backward compatibility is maintained, the following steps should be taken:
+- Given `X_Y_ZZ` is the version of the OpenAPI before the change:
+    - Create a copy of the previous version structure as it was before the backward-compatible change, suffixed with the last version of the OpenAPI before the change, e.g., `CertificateMessageUntilV0_1_32`.
+    - Implement a `golden_message_until_open_api_X_Y_ZZ` method that returns the representation of the `CURRENT_JSON` using the previous structure.
+    - Implement a `test_current_json_deserialized_into_message_supported_until_open_api_X_Y_ZZ` test that checks that deserializing the `CURRENT_JSON` into the previous structure yields the output stored in `golden_message_until_open_api_X_Y_ZZ`.
+
+When modifying an existing JSON message structure, if backward compatibility is not maintained, the following steps should be taken:
+- Modify the `CURRENT_JSON` string to reflect the new structure.
+- Modify the `golden_message_current` method to return the representation of the `CURRENT_JSON` using the new structure.
+- Remove all `golden_message_until_open_api_X_Y_ZZ` method and the corresponding structure and tests, as they are no longer relevant.
+
+## Consequences
+
+- Ensures that any changes to the JSON message structure are backward compatible.
+- Provides a clear and standardized approach to testing JSON message deserialization.
+- Helps maintain the integrity and reliability of the communication between nodes.
+- Requires maintaining multiple versions of message structures and corresponding tests, which may increase the maintenance overhead.