add async no threading flag

[Hynek Kydlíček (hynky1999)]

Summary

• add an ASYNC_NO_THREADING debug flag for pipeline blocking offloads
• route video commit, decoder cache opens, and media cache downloads through one helper
• document how to run the flag locally

Testing

• uv run ruff check src/refiner/execution/asyncio/runtime.py docs/local-execution.md src/refiner/pipeline/sinks/lerobot/_lerobot_video_writer.py src/refiner/pipeline/utils/cache/decoder_cache.py src/refiner/pipeline/utils/cache/file_cache.py
• `uv run python - <<'PY'
  import os, asyncio
  from refiner.execution.asyncio.runtime import run_in_io_executor

async def main():
os.environ["ASYNC_NO_THREADING"] = "1"
value = await run_in_io_executor(lambda: "ok")
print(value)

asyncio.run(main())
PY`

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the asynchronous execution model by introducing a new debug flag, ASYNC_NO_THREADING. This flag allows developers to bypass threadpool offloads for specific blocking I/O operations, enabling easier debugging of async code. The change also centralizes the execution of these operations through a new helper function, run_in_io_executor, improving consistency and maintainability across various pipeline components.

Highlights

• New Debug Flag: Introduced ASYNC_NO_THREADING environment variable to disable threadpool offloads for blocking I/O operations, forcing them to run inline on the async runtime thread.
• Centralized Async Execution: Consolidated video commit, decoder cache opens, and media cache downloads to use a new run_in_io_executor helper.
• Documentation: Added documentation for the ASYNC_NO_THREADING flag, explaining its purpose and how to use it for local debugging.

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[gemini-code-assist]

Code Review

This pull request introduces a useful ASYNC_NO_THREADING debug flag and centralizes the logic for offloading blocking calls into a run_in_io_executor helper. The refactoring across the different files to use this new helper is clean and improves consistency. I've found one potential issue with how the environment variable is checked, which could lead to unexpected behavior. My feedback is focused on making this check more robust.

[gemini-code-assist]

The current check for ASYNC_NO_THREADING is truthy for any non-empty string. This means if a user sets ASYNC_NO_THREADING=0 or ASYNC_NO_THREADING=false expecting to disable this debug mode, it will actually be enabled because "0" and "false" are truthy as strings. This can be misleading.

To make the behavior more explicit and align with common conventions for boolean flags in environment variables, it's better to check for a specific value like "1". The documentation already instructs users to set it to 1.

Suggested change

	if os.environ.get("ASYNC_NO_THREADING"):
	if os.environ.get("ASYNC_NO_THREADING") == "1":