Minor readability cleanups in `oak_functions_service` #12649

Job	Run time
xtask (format)	6m 35s
xtask (check-format)	6m 23s
xtask (run-cargo-deny)	5m 28s
xtask (run-cargo-udeps)	11m 1s
xtask (run-bazel-tests)	6m 45s
xtask (run-cargo-fuzz -- -max_total_time=2)	5m 37s
xtask (completion)	6m 19s
xtask (run-cargo-clippy)	15m 36s
	1h 3m 44s

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