feat(isthmus): observe scalar function types

[alexandrefimov]

Summary

• add an opt-in type observer for Substrait-to-Calcite scalar function conversion
• report the source expression, supplied Substrait type, and either the independently inferred Calcite type or the inference failure
• expose observer configuration through ConverterProvider while keeping the default conversion path unchanged

Related to #379.

Behavior

• observations are emitted for both matching and differing scalar function types
• the converted RexNode continues to use the type supplied by Substrait
• Calcite inference failures are reported without turning an otherwise successful conversion into a failure
• exceptions thrown by the observer are propagated to the caller
• variadic CONCAT produces one observation for the complete invocation

This PR intentionally covers scalar functions only. Window functions, field references, and aggregate functions remain separate follow-up work.

Validation

• ./gradlew build --rerun-tasks
• focused scalar observation tests covering matches, mismatches, inference failures, nested calls, provider injection, observer exceptions, and variadic CONCAT

Assisted-by: gpt-5.6-sol (OpenAI Codex)

[CLAassistant]

All committers have signed the CLA.

[nielspardon]

@vbarua can you take a look since you originally opened #379?

[vbarua]

Did a quick pass and the direciton seems reasonable overall. Planning on taking a closer look tomorrow.

[vbarua]

Left some minor comments, but looks good overall.

[alexandrefimov]

Thanks for the review!
The five minor comments are clear.
I’ll address them together with the CONCAT simplification after #1025 is merged.

[vbarua]

CONCAT simplification is merged 🙂

[alexandrefimov]

@vbarua rebased on main and addressed all review comments. Ready for another look when you have a chance. Thanks!