fix(isthmus): support variadic concat conversion

[bvolpato]

Summary

Convert variadic Substrait concat expressions into left-associated binary Calcite calls. This preserves argument order while satisfying Calcite || operand-count validation.

Closes #724.

Changes

• Fold concat calls with more than two arguments into nested binary calls.
• Keep existing two-argument conversion unchanged.
• Add regression coverage for operator shape and operand order.

Repro

Converting concat("a", "b", "c") previously passed three operands to Calcite || and failed with wrong operand count 3 for ||. It now produces concat(concat("a", "b"), "c").

Testing

• ./gradlew :isthmus:test --tests io.substrait.isthmus.FunctionConversionTest
• ./gradlew spotlessApply
• ./gradlew build --rerun-tasks

Local integrationTest could not start because no Docker daemon is available; GitHub Actions runs that Testcontainers suite.

Did this cause any problems?

No. Revert this commit to restore previous conversion behavior.

[nielspardon]

LGTM, the only issue this introduces is that we can not cleanly round-trip plans Substrait -> Calcite -> Subtrait if the initial Substrait plan contained a variadic concat function call. I don't think there is an elegant way to solve this during conversion and maybe should be considered in the semantic-aware plan comparison issue #994