Summary

This PR introduces support for optional decimal rounding in the DecimalField serializer field. By adding a rounding parameter, developers can now control how decimal values are quantized during serialization — such as rounding up, down, or to the nearest value.

Motivation

Controlling decimal precision is crucial in real-world APIs that handle:

• 💸 Financial calculations (e.g., prices, taxes, discounts)
• 📏 Measurement tools (e.g., distances, weights)
• 📊 Scientific data and reports

While DecimalField enforces digit and decimal precision, it lacked direct control over the rounding mode. This enhancement improves the flexibility and precision of API responses for such use cases.

What's Changed

• 🔧 Added a new rounding parameter to DecimalField.__init__()
• 🧮 Applied rounding logic in to_representation() using Python's Decimal.quantize()
• 🔒 Added validation to ensure rounding is one of the decimal.ROUND_* constants
• ✅ Added a dedicated unit test in tests/test_fields.py to verify rounding behavior (e.g., ROUND_HALF_UP)

Example Usage

from decimal import ROUND_HALF_UP

price = serializers.DecimalField(
    max_digits=6,
    decimal_places=2,
    rounding=ROUND_HALF_UP
)

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## Backward Compatibility

✅ The new parameter is **optional**  
✅ If `rounding` is not provided, existing behavior is unchanged  
✅ Fully backward-compatible with all existing serializers

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## Testing

- All core DRF tests pass using `pytest`  
- Custom test added for rounding behavior in `tests/test_fields.py`

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## Additional Notes

- Contribution aligns with DRF’s goal of being a flexible, production-grade API toolkit  
- This feature has real-world demand across finance, commerce, and analytics

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Please let me know if any changes or enhancements are needed!