Add stronger type annotations in file sources + refactoring #26646

Job	Run time
setup-selenium / setup_chromedriver	5s
build-client / build-client	11m 1s
Test (3.8, 0)	41m 53s
Test (3.8, 1)	52m 38s
Test (3.8, 2)	49m 50s
	2h 35m 27s

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