🧪 [testing improvement] Add tests for _parameters_by_id in diff.py

[edithatogo]

🎯 What: The testing gap addressed
The _parameters_by_id function in contracts/tools/src/pic_contracts/diff.py lacked explicit test coverage. This function takes a document dictionary and returns a dictionary mapping parameter IDs to parameter dictionaries.

📊 Coverage: What scenarios are now tested

1. Happy Path: Correctly mapping multiple parameters by their IDs.
2. Missing Key: Handling a document without a "parameters" key (returns an empty dictionary).
3. Empty List: Handling a document with an empty "parameters" list (returns an empty dictionary).
4. Missing ID: Ensuring a KeyError is raised if a parameter dictionary is missing the "id" key.

✨ Result: The improvement in test coverage
The _parameters_by_id function is now fully covered by tests, ensuring its behavior remains correct and deterministic. This prevents regressions if the way parameter IDs are extracted or mapped is changed in the future.

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PR created automatically by Jules for task 381276396441711887 started by @edithatogo

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