JsonValue: Deduplicate keys before populating Dict

[friendlymatthew]

Change Summary

This PR deduplicates keys in JsonValue::as_kwargs before populating a PyDict to avoid creating Python objects for values that will be overwritten by duplicate keys

[codspeed-hq]

CodSpeed Performance Report

Merging #1865 will degrade performances by 11.39%

_{Comparing friendlymatthew/dedup-keys-before-building-dict (2bbac0c) with main (f892755)}

Summary

❌ 1 regression
✅ 162 untouched

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	test_set_of_ints_core_json_duplicates	382.7 µs	431.9 µs	-11.39%

[davidhewitt]

Thanks 🎉