Push down compatible UNION queries and grouped set operations

[k-bx]

Summary

Add foreign-path and deparsing support for compatible PostgreSQL set-operation relations.

Planner changes

• recognize set-operation arms backed by compatible pg_clickhouse foreign paths
• require the same foreign server and effective user mapping for every arm
• deparse compatible UNION ALL and UNION DISTINCT trees as a single remote query
• preserve target-column positions and renumber parameters across arms
• support conservative plain DISTINCT pushdown
• allow compatible grouping and aggregation above flattened UNION ALL relations
• allow inlined CTEs to participate without changing materialized CTE behavior
• reject unsupported types, table engines, ordering, HAVING, mixed servers, local conditions, and parameterized paths

Unsupported shapes are not given a foreign path and retain their existing planner behavior.

Validation

• project pre-commit checks
• clean PostgreSQL 18 build with warnings treated as errors
• focused planner and execution regressions for binary and HTTP drivers
• real-world production canary validation with a multi-table analytical workload using UNION ALL and grouped COUNT(DISTINCT ...); verified result parity, remote execution of the combined query, and stable repeated execution on production-scale data