Notebook: not override cell language settings

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We have move some cell language unit tests to integration tests. Currently, it falls back to default SQL kernel. So the notebook level language is overwritten by current logic.

We fix the code and not not override cell language settings in the notebook. Once we have product fix, we can enable 3 cellLanguageTests.

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Thanks for submitting this issue. Please also check if it is already covered by an existing one, like:

• Notebooks: add cell indicator when default language is overridden (#4079)
• Notebooks: add test for language detection in ipynb files (#4094)