fix(openai): correct image resize aspect ratio caps

[towseef41]

Summary

• fix _resize in langchain_openai.chat_models.base so the smaller side is scaled with its own dimension, preserving aspect ratio (e.g., 2048x4096 -> 768x1536)
• add a regression test covering both orientations to prevent regressions

Fixes #34191

Testing

• OPENAI_API_KEY=dummy python3 -m uv run --group test pytest tests/unit_tests/chat_models/test_base.py -k resize
• python3 -m uv run --group lint ruff format langchain_openai/chat_models/base.py tests/unit_tests/chat_models/test_base.py
• python3 -m uv run --group lint ruff check langchain_openai/chat_models/base.py tests/unit_tests/chat_models/test_base.py

[towseef41]

@ccurme waiting for your review

[codspeed-hq]

CodSpeed Performance Report

Merging #34192 will not alter performance

_{Comparing towseef41:fix/resize-aspect-ratio (75065e4) with master (80c3970)}

Summary

✅ 6 untouched
⏩ 28 skipped¹

Footnotes

1. 28 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

thought more

[ccurme]

Do we have a usage example that demonstrates that something is broken or wrong?

[towseef41]

Do we have a usage example that demonstrates that something is broken or wrong?

The resize bug is covered by libs/partners/openai/tests/unit_tests/chat_models/test_base.py::test__resize_caps_dimensions_preserving_ratio. It calls _resize with oversized dimensions (2048x4096 and 4096x2048) and asserts the capped sizes keep the aspect ratio (768x1536 and 1536x768). That test is the usage example demonstrating the prior incorrect scaling and validating the fix.