⚡ Bolt: Add fast path for single-batch build side in HashJoinExec

[google-labs-jules]

This pull request introduces a performance optimization to the HashJoinExec by adding a fast path for single-batch build sides.

💡 What: The optimization adds a condition to check if the build side of a hash join consists of a single RecordBatch. If so, it bypasses the expensive concat_batches operation and builds the hash map directly from that single batch.

🎯 Why: The concat_batches function is a known performance bottleneck as it involves allocating new memory and copying data. By avoiding this operation in the common single-batch case, we can significantly reduce memory allocation and CPU usage.

📊 Impact: This change improves the performance of hash joins where the build side is small enough to fit into a single batch.

🔬 Measurement: The performance improvement can be verified by running benchmarks that involve hash joins with single-batch build sides. The existing tests in datafusion/physical-plan/src/joins/hash_join/exec.rs cover this new path and continue to pass.

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PR created automatically by Jules for task 6592070358591634336 started by @Dandandan

[google-labs-jules]

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