Fix background mode auto-enable for long requests

[hemanth-asirvatham]

Motivation

• Ensure get_response auto-enables background polling for long-running requests when no timeout is provided, matching the documented behavior instead of defaulting to False.

Description

• Update in src/gabriel/utils/openai_utils.py where effective_background is now set to timeout is None rather than False, so background mode is automatically used when callers omit a timeout.

Testing

• No automated tests were run for this change.

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Codex Task

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