add benchmark for root model validation

[davidhewitt]

Change Summary

Related to #1580, will merge this first.

Related issue number

N/A

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1581 will not alter performance

_{Comparing dh/root-model-benchmark (ce824d0) with main (80756c1)}

Summary

✅ 155 untouched benchmarks
🆕 2 new benchmarks

Benchmarks breakdown

	Benchmark	main	dh/root-model-benchmark	Change
🆕	test_complete_core_root	N/A	274.8 µs	N/A
🆕	test_complete_core_root_json	N/A	666.1 µs	N/A