🧪 [testing improvement] Cover SmartMemory.active_topics

[dioncx]

What: Addressed the testing gap for the active_topics property in SmartMemory.
Coverage: The test covers empty state initialization, correct topic extraction and formatting (lowercasing), ordered insertion, and the 10-item limit.
Result: Increased test coverage and guarantees over the active topics feature which is responsible for returning the contextual topics for hybrid fallback scoring.

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PR created automatically by Jules for task 13952044932135806248 started by @dioncx

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