fix(clickhouse): implement re_scan for nested loop joins

[JohnCari]

Summary

• Fixes [clickhouse_fdw] Data is missing when joining two foreign tables #532

PostgreSQL uses Nested Loop joins when joining foreign tables. During a Nested Loop join, the inner (right) table is scanned multiple times - once for each row of the outer table. The FDW framework calls re_scan() to restart the inner scan for each outer row.

The ClickHouse FDW was using the default no-op re_scan() implementation, which caused joins to return incomplete results because after the first scan:

• is_scan_complete was still true
• row_receiver channel was exhausted
• No new streaming task was spawned

This fix implements re_scan() to restart the async streaming by:

• Aborting existing streaming task if running
• Resetting scan state flags
• Reinitializing the bounded channel
• Spawning new streaming task with the same SQL query
• Fetching the first row to initialize the rescan

Test plan

• Added integration test clickhouse_join_test that verifies inner, left, and right joins return correct results
• All existing tests pass (cargo pgrx test pg16)
• cargo clippy passes with no warnings
• cargo fmt passes

The test creates two foreign tables with values (1, 2, 3) each and verifies:

• Inner join returns 3 rows with values (1,1), (2,2), (3,3)
• Left join returns 3 rows
• Right join returns 3 rows