fix: process_video silent hang on callback error

[realh4m]

Description

Added exception tracking in process_video so callback errors raised in the worker loop are surfaced after reader/writer threads finish, preventing silent hangs when the callback crashes mid-stream.

Introduced read_finished to avoid draining the read queue unnecessarily once EOF is reached.

• Issue: Fixes the silent failure where process_video could hang without any error when the callback raised an exception.
• Dependencies: None.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How has this change been tested, please provide a testcase or example of how you tested the change?

Verified locally with a callback that raises an exception.

Any specific deployment considerations

No additional considerations; behavior change only affects process_video error handling.

Docs

• Docs updated? What were the changes:

[CLAassistant]

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