🧪 Add tests for AdvancedSearch.full_text_search

[dioncx]

🎯 What: Adds comprehensive unit tests for AdvancedSearch.full_text_search which previously lacked dedicated test coverage.
📊 Coverage: Validates behavior of multi-word searches, empty queries, case-sensitivity, correct score calculation for path vs. value matches, and edge cases like None or non-string values within the JSON memory.
✨ Result: Substantially increases code coverage for json_memory/search.py and provides a reliable safety net against future regressions.

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PR created automatically by Jules for task 5366755400262804350 started by @dioncx

[google-labs-jules]

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