[BUG] Fix get_flow_run_log_content 400 error when log file is in creation

[Copilot]

Problem

When running pf.runs.create_or_update() from the PromptFlow SDK and immediately trying to access logs through functions that call get_flow_run_log_content, users sometimes encounter a 400 error with the message "Value cannot be null. (Parameter 'bytes')". This happens when the log file for the run is still in the process of being created, causing the first access attempt to fail.

Solution

Added a retry mechanism specifically for this scenario in the _get_log method of RunOperations. The implementation:

1. Adds a specialized retry decorator around the get_flow_run_log_content call
2. Only retries on 400 errors that contain the specific error message "Value cannot be null. (Parameter 'bytes')"
3. Uses exponential backoff starting at 1 second, doubling each time
4. Makes up to 5 retry attempts before giving up
5. Logs a warning message during retries to inform the user
6. Maintains original behavior for any other types of errors

Example usage pattern that now works correctly

from promptflow.azure import PFClient

# Connect to workspace
pf = PFClient(workspace=ws)

# Create a new run
run = pf.runs.create_or_update(...)

# Immediately stream logs - previously would fail with 400 error
# Now retries until log file is created
pf.stream(run)

With this change, users no longer need to wait or add manual delays before accessing logs for newly created runs.

Fixes #4002.

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[github-actions]

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