After #2226.

https://www.mongodb.com/docs/v6.0/reference/operator/aggregation/sum/

DoD:

• only for $group stage (as everything else is not implemented yet)
• consider overflow cases
• regular test cases: $v already includes non-mixed (only numeric) and mixed cases (e.g. numeric + non-numeric, arrays etc)
• constant test case: ({ $sum: 1 })