[CALCITE-7440] Preserve correlated scope mapping in RelToSqlConverter

[bvolpato]

Summary

RelToSqlConverter failed during PostgreSQL round-trip validation when semi-join rewrites introduced a correlated filter with an unresolved correlation id.

Reproduction query

WITH product_keys AS (
  SELECT p."product_id",
         (SELECT MAX(p3."product_id")
          FROM "foodmart"."product" p3
          WHERE p3."product_id" = p."product_id") AS "mx"
  FROM "foodmart"."product" p
)
SELECT DISTINCT pk."product_id"
FROM product_keys pk
LEFT JOIN "foodmart"."product" p2 USING ("product_id")
WHERE pk."product_id" IN (
  SELECT p4."product_id"
  FROM "foodmart"."product" p4
)

Error before this PR

java.lang.NullPointerException: variable $cor1 is not found
  at org.apache.calcite.rel.rel2sql.SqlImplementor.getAliasContext(SqlImplementor.java:1590)

Fix

Register missing RexCorrelVariable ids while visiting filter conditions, without overriding existing correlation mappings.

Test

• Added RelToSqlConverterTest.testPostgresqlRoundTripCorrelatedProjectWithSemiJoinRules
• Ran: ./gradlew :core:test --tests org.apache.calcite.rel.rel2sql.RelToSqlConverterTest
• Result: 563 passed, 0 failed

Disclaimer

The changes were done with the assistance of Codex (gpt-5.3-codex) in response to an actual bug when using Calcite in production. The changes were manually verified prior to sending this to review.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
33.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud