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]

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