fix: use monotonic clock for file processing timeout - #3748

Summary

• Use time.monotonic() for elapsed-time deadlines in both synchronous and asynchronous file-processing polling.
• Add regression coverage for a wall-clock rollback in both resource variants.

Bug

Files.wait_for_processing() and its asynchronous counterpart used time.time() to measure elapsed time. If the system wall clock moves backwards while a file is still processing, the calculated elapsed time also moves backwards and the timeout can be postponed indefinitely.

Reproduction

With max_wait_seconds=1, poll_interval=5, and a wall clock that changes from 100 to 90 after the first poll:

• Expected: raise the timeout error once the monotonic deadline has elapsed.
• Actual before this change: continue polling and enter another five-second sleep.

Root cause

Wall-clock time is adjustable and is not suitable for measuring elapsed durations.

Fix

Use Python's monotonic clock for both the initial timestamp and every deadline check. This does not change the public API, poll interval, terminal states, or error message.

Validation

• tests/lib/test_file_processing.py: 15 passed
• Ruff check: passed
• Ruff format check: passed
• Isolated synchronous and asynchronous rollback reproducer: passed

Commands used:

PYTHONPATH=/tmp/openai-python-file-timeout-deps-20260828:src python -m pytest -p no:cacheprovider -o addopts= --confcutdir=tests/lib --asyncio-mode=auto -q tests/lib/test_file_processing.py
python -m ruff check --no-cache src/openai/lib/_files.py tests/lib/test_file_processing.py
python -m ruff format --no-cache --check src/openai/lib/_files.py tests/lib/test_file_processing.py