🧪 [Testing Improvement] Add comprehensive tests for trace_equivalence

[edithatogo]

🎯 What: Extracted the existing test for trace_equivalence from test_pic_traces_schema.py into its own dedicated file test_traces.py and expanded it.
📊 Coverage: The new tests now completely check the returned dictionaries, particularly verifying that the diffs array correctly contains the right levels and messages when output, path, or semantic diffs occur. It also covers empty trace cases and missing keys.
✨ Result: Enhanced test coverage that explicitly checks error messages and edge cases, solidifying confidence in the trace equivalence logic without changing functionality.

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PR created automatically by Jules for task 18270784420901342797 started by @edithatogo

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