feat: testing] Test PooledEmbedding.mean_pooling

[n24q02m]

🎯 What: The class method mean_pooling in PooledEmbedding lacked unit tests. This method wraps mathematical functionality mapping unmasked tokens arrays to mean pooled representations.

📊 Coverage:

• Happy path: Tests standard batch processing of a typical 3D numpy array representing sequential data with varying attention_mask values.
• Empty mask: Tests passing an all-zero attention_mask, verifying the division-by-zero avoidance strategy.
• Varying lengths: Tests a batch where sequence tokens valid/unmasked lengths vary to verify calculations happen accurately.

✨ Result: Test coverage for PooledEmbedding is vastly improved and mathematical accuracy of mean_pooling in edge cases is safely protected against future regressions.

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PR created automatically by Jules for task 7521944429730682362 started by @n24q02m

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[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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