⚡ perf: Optimize high concurrency test by wrapping payload in Arc

[panayang]

💡 What:
Replaced Vec<u8> with Arc<Vec<u8>> for the encoded payload in tests/high_concurrency.rs. This involved updating PartialReader's data field and run_worker's argument type, along with wrapping the serialized payload in an Arc inside both test functions.

🎯 Why:
The high concurrency test was performing a deep clone of the encoded payload vector inside the task spawning loop (encoded.clone()). In a tight loop spawning 5,000,000 tasks, this leads to massive, expensive memory allocations that drastically affect performance. Wrapping the payload in an Arc (Atomic Reference Count) ensures only the reference count is incremented upon cloning, avoiding deep copies of the byte array and saving significant CPU and memory allocation overhead.

📊 Measured Improvement:
Running cargo test --release --test high_concurrency before and after the change showed significant performance improvements:

Baseline:

• Batching: Processed 5000000 tasks in 8.17s (612166.94 tasks/sec)
• No Batching: Processed 5000000 tasks in 23.58s (212024.57 tasks/sec)

After Optimization:

• Batching: Processed 5000000 tasks in 5.67s (881996.51 tasks/sec) -> ~44% faster
• No Batching: Processed 5000000 tasks in 7.85s (637342.51 tasks/sec) -> ~200% faster

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PR created automatically by Jules for task 13273604940859237615 started by @panayang

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