fix(isthmus): std_dev, variance function mappings

[nielspardon]

Disclaimers upfront: I have been assisted by AI generating parts of the code, documentation and PR description.

This PR still uses the to be deprecated function signatures using options. I will migrate the code to enum arguments in a follow-up.

Since this PR also casts the input arguments to FP64 if necessary, we do not need the integer based function signatures proposed in substrait-io/substrait#1012

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This PR fixes the bidirectional conversion between Calcite and Substrait for statistical aggregate functions (standard deviation and variance). Previously, these functions were not properly mapped, preventing SQL queries using STDDEV_POP, STDDEV_SAMP, VAR_POP, and VAR_SAMP from being correctly converted to Substrait and back.

Problem

Substrait uses a single function name for both population and sample variants of statistical functions:

• std_dev for both STDDEV_POP and STDDEV_SAMP
• variance for both VAR_POP and VAR_SAMP

The distinction between population (n denominator) and sample (n-1 denominator) variants is made via a distribution function option (POPULATION or SAMPLE), but this mechanism was not implemented in the Isthmus converter. The tests did pass prior to this change since the existing TPC-DS testcases using these functions were incorrectly mapped to the AVG function since Calcite maps these statistical functions to SqlAvgAggFunction.

Solution

Calcite → Substrait Direction:

• Added function mappings in FunctionMappings for all four statistical operators
• Enhanced AggregateFunctionConverter.generateBinding() to automatically inject the distribution option based on the Calcite SqlKind:
  • STDDEV_SAMP, VAR_SAMP → distribution=SAMPLE
  • STDDEV_POP, VAR_POP → distribution=POPULATION

Substrait → Calcite Direction:

• Extended FunctionConverter.getSqlOperatorFromSubstraitFunc() to accept function options as a parameter
• Implemented filterByFunctionOptions() to disambiguate operators using the distribution option
• Updated SubstraitRelVisitor to pass function options during conversion

DSL Support:

• Added convenience methods in SubstraitBuilder: stddevPopulation(), stddevSample(), variancePopulation(), varianceSample()
• These methods automatically configure the appropriate distribution option