feat(isthmus): observe window function return types

[alexandrefimov]

Summary

• report Calcite-inferred return types through TypeObserver during window-function conversion
• preserve the Substrait-supplied return type and skip extra inference for TypeObserver.NOOP
• allow callers to configure the observer through ConverterProvider.Builder

Motivation

Scalar-function type observation was introduced in #1015. Issue #379 also identifies window functions as a conversion point where Substrait-supplied and Calcite-inferred types can diverge. This change extends the same observation side channel to window functions and makes it available without subclassing ConverterProvider.

Validation

• ./gradlew :isthmus:spotlessCheck :isthmus:test --tests 'io.substrait.isthmus.ConverterProviderBuilderTest' --tests 'io.substrait.isthmus.SubstraitExpressionConverterTest' :isthmus:javadoc
• ./gradlew build --rerun-tasks

Related to #379.

[alexandrefimov]

@vbarua this follows up on #1015 — it extends type observation to window functions (the second conversion point named in #379) and wires the observer into ConverterProvider.Builder, which is what you suggested after the merge. Would you have a chance to take a look?

[vbarua]

Overall, the changes look reasonable.

There is one potential issue around nullability in the Calcite inferred output types. From a cursory look, I don't know how painful it would be to have the observation path follow the true Calcite inference path more closely. If it's not too bad, it would be worth doing in this PR. However, if it would require a fair bit of poking into Calcite internals to achieve, I would be happy with documenting the limitation for now and merging.

Let me know what you think @alexandrefimov

[vbarua]

Claude did identify one potential bit of weirdness around nullability differences in certain cases. I still think this is useful overall for cases where the types are fully different, but it might not be reliable as-is to detect nullability deviations.

Claude Summary

rexBuilder.deriveReturnType(operator, args) wraps args in a plain RexCallBinding where hasEmptyGroup() always returns false.

Calcite's actual window inference path (via SqlOverOperator) sets hasEmptyGroup() based on whether the window bounds guarantee non-emptiness. For SUM, MIN/MAX, AVG, and several other aggregate window functions, this flag controls result nullability.

Concrete example: SUM(x BIGINT NOT NULL) OVER (...) — the real Calcite inference returns BIGINT nullable; deriveReturnType returns BIGINT NOT NULL. The observation records the wrong inferred type, defeating the diagnostic purpose of the observer.