fix: Force CollectLeft in physical planner for null-aware joins

Added an additional check in the physical planner to prevent null-aware
anti joins from using PartitionMode::Auto. This ensures they use
PartitionMode::CollectLeft from the start, before any optimizer passes.

The issue: Even with the fix in join_selection.rs, the physical planner
was creating null-aware joins with PartitionMode::Auto when
target_partitions > 1 and repartition_joins is enabled (common in CI).

The fix: Added `&& !*null_aware` condition to the partition mode
decision in the physical planner, forcing null-aware joins to skip
the Auto mode and go directly to CollectLeft.

This provides defense-in-depth:
1. Physical planner: Creates with CollectLeft initially
2. Join selection optimizer: Ensures it stays CollectLeft
3. Stream execution: Has per-partition tracking as backup

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: viirya

datafusion/core/src/physical_planner.rs

@@ -1488,6 +1488,7 @@ impl DefaultPhysicalPlanner {
                 } else if session_state.config().target_partitions() > 1
                     && session_state.config().repartition_joins()
                     && prefer_hash_join
+                    && !*null_aware  // Null-aware joins must use CollectLeft
                 {
                     Arc::new(HashJoinExec::try_new(
                         physical_left,