feat(isthmus): map execution context variables to/from Calcite

[nielspardon]

Summary

Adds Isthmus (Substrait ⇄ Apache Calcite) mappings for the three execution context variables introduced in :core by #867: current_timestamp, current_date and current_timezone. Previously any plan containing one of these failed to convert to/from Calcite.

Changes

• Substrait → Calcite (ExpressionRexConverter): three visit overrides build niladic RexCalls whose return type is forced from the Substrait type so time-zone semantics, precision and required-nullability round-trip.
  • CurrentTimestamp(p) → CURRENT_TIMESTAMP typed TIMESTAMP_WITH_LOCAL_TIME_ZONE(p) — Substrait's current_timestamp is a precision_timestamp_tz, whereas Calcite's operator would otherwise infer a timezone-less TIMESTAMP.
  • CurrentDate → CURRENT_DATE typed DATE.
  • CurrentTimezone → the new CurrentTimezoneFunction typed non-nullable VARCHAR.
• Calcite → Substrait: new CallConverters.EXECUTION_CONTEXT_VARIABLE matches these operators by identity and produces the corresponding Substrait expressions. Registered in both CallConverters.defaults() and ConverterProvider.getCallConverters().
• Session timezone operator: Calcite has no built-in operator for the session timezone, so this adds CurrentTimezoneFunction — a niladic SqlBaseContextVariable (modeled on Calcite's own CURRENT_USER/CURRENT_ROLE). It is registered in SubstraitOperatorTable via a separate scalar-operator table so that:
  • SELECT current_timezone parses from SQL text without parentheses (via Calcite's SqlValidatorImpl.makeNullaryCall), and
  • its generic OTHER_FUNCTION kind does not pollute OVERRIDE_KINDS (which would otherwise filter every standard OTHER_FUNCTION operator out of the operator list).

Tests

• ExecutionContextVariableConversionTest: bidirectional expression ⇄ Rex round-trips for all three variables.
• Substrait2SqlTest: full SQL round-trips for current_timestamp, current_date, and current_timezone, including asserting that current_timezone parses and is emitted back as the bare keyword (no Calcite dialect changes required).

Verification

:core:spotlessCheck, :isthmus:spotlessCheck, the full :isthmus:test suite, and compilation of :spark:spark-3.5_2.12 and :examples:substrait-spark all pass.

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