Add optional on_check callback to Job.join()

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Summary

• Add an optional on_check callback parameter to Job.join() that is called after each polling iteration with (elapsed_time, status)
• Enables progress logging, progress bars, or custom monitoring during the blocking join() call
• Fully backward compatible — defaults to None, no behavior change if not provided

Usage

# Simple progress logging
job.join(on_check=lambda elapsed, status: print(f"Waiting... {elapsed}s ({status})"))

# Or with a progress bar
from tqdm import tqdm
bar = tqdm(total=300, unit="s")
job.join(on_check=lambda elapsed, status: bar.update(3))

Test plan

• Callback is invoked on each non-ready polling iteration with correct elapsed time and status
• No error when on_check is not provided (backward compatibility)
• Callback is not called when task is immediately ready

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