fix: Improve `Accept.best_match` typing #6283

Job	Run time
validate	17s
mypy	1m 6s
pyright	38s
slotscheck	19s
test_typing_extensions (4.12.1)	19s
test_typing_extensions (latest)	17s
test_typing_extensions (4.13.1)	21s
Test Minimal Application with Base Dependencies	17s
Test Minimal Pydantic 1 application	26s
Test server integration (uvicorn<0.27.0)	46s
Test server integration (uvicorn>=0.27.0)	54s
test-platform-compat	0s
test (3.13) / test	1m 48s
test (3.9) / test	2m 19s
test (3.10) / test	1m 44s
test (3.11) / test	1m 30s
test (3.12) / test	2m 39s
build-docs	2m 44s
test_success	3s
upload-test-coverage	21s
codeql	2m 14s
	21m 2s

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