[#723] Fix inconsistent farmer distribution in IDC auto segmentation

[wayangalihpratama]

Summary

This PR addresses numerical segmentation inconsistencies by implementing new strategies to ensure a more balanced distribution of entities across all segments.

Key Changes

• Numerical Segmentation: Switched to non-interpolating quantiles (closest_observation) to eliminate empty segments and artificial thresholds caused by data gaps.
• New Strategies: Added support for Equal Interval strategy in backend segmentation logic for better interval-based grouping.
• API Updates: Updated Pydantic models and routes to support optional strategy selection in segmentation previews and recalculations.
• Unit Tests: Added backend/tests/test_unit_segmentation_strategy.py for comprehensive coverage of segmentation strategies, including quantile and equal interval methods.

Verification

• Ran backend unit tests: test_unit_segmentation_strategy.py successfully verified quantile and equal interval strategies.

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