Fix uncertainty calculation in StereoMeanCombiner #5011

Job	Run time
lint	13s
docs	8m 57s
tests (Linux (3.10, mamba), ubuntu-latest, 3.10, mamba, tests,all)	12m 9s
tests (Linux (3.10, pip), ubuntu-latest, 3.10, pip, tests,all)	12m 35s
tests (Linux (3.11, pip, minimal-dependencies), ubuntu-latest, 3.11, pip, tests)	11m 43s
tests (Linux (3.12, mamba, coverage), ubuntu-latest, 3.12, mamba, codecov, tests,all)	13m 6s
tests (Linux (3.12, pip), ubuntu-latest, 3.12, pip, tests,all)	16m 27s
tests (macos (3.10, x86_64, pip), macos-13, 3.10, pip, tests,all)	16m 42s
tests (macos (3.12, arm64, mamba), macos-14, 3.12, mamba, tests,all)	11m 51s
	1h 43m 43s

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