isthmus: honor concat IGNORE_NULLS and like CASE_INSENSITIVE by selecting a Calcite operator from the options #1401

Once #1390 lands, isthmus maps each covered string function to one Calcite operator and rejects any option preference that operator does not implement. That is spec-correct, but Calcite already has operators for two of the rejected behaviors:

• concat with null_handling: [IGNORE_NULLS]: SqlLibraryOperators.CONCAT_FUNCTION_WITH_NULL treats a null argument as the empty string, which is what the IGNORE_NULLS cases in the spec's tests/cases/string/concat.test expect, including concat(null, null) = ''.
• like with case_sensitivity: [CASE_INSENSITIVE]: SqlLibraryOperators.ILIKE.

scalar_function concat:str(string 'a', string? null) options { null_handling: [IGNORE_NULLS] }
// after #1390: UnsupportedOperationException ("Calcite concat requires null_handling ACCEPT_NULLS")

Proposal: let the option policy pick the operator from the preference list on import, and emit IGNORE_NULLS / CASE_INSENSITIVE when exporting those operators. That would make the policy a choice between operators, the way the distribution enum argument already chooses between STDDEV_POP and STDDEV_SAMP.

Things to check before binding library operators:

• Nullability: Calcite types CONCAT_FUNCTION_WITH_NULL's result NOT NULL, whereas the spec's concat test types the result str?.
• Known Calcite traps: binding a SqlLibraryOperators constant directly has previously broken ORDER BY, emitted a widened output_type, and failed in the native image. A local operator registered in SUBSTRAIT_SCALAR_OPERATOR_TABLE may be needed instead.

Follow-up from the #1390 review; related to #1173 and #1012.