ci: bump codecov/codecov-action from 4 to 6 #15

Job	Run time
Test (Python 3.13 free-threaded, experimental)	15s
Performance Regression Gate	21s
Type check	17s
Test (Python 3.13)	20s
Test (Python 3.12)	23s
Lint	11s
Memory Budget Check	16s
	2m 3s

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