Improve test coverage in `datafusion-proto`

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
The `datafusion-proto` crate provides serde of logical plans and expressions. We have limited test coverage and it would be good to have more comprehensive tests.

**Describe the solution you'd like**
Write tests to cover all operators and expressions. We could also consider using fuzz testing as described in https://howqueryengineswork.com/14-testing.html

**Describe alternatives you've considered**
None

**Additional context**
None

Start by locating the datafusion-proto crate and its existing tests for serde of logical plans and expressions. Inventory the operators and expressions that lack coverage, then add tests until they are comprehensively exercised; fuzz testing is an optional direction described in the linked resource.

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