[Bugfix] Enforce --max-generated-image-size on /v1/images/generations

[NickCao]

Purpose

The /v1/images/edits endpoint validates the requested image size against --max-generated-image-size, but /v1/images/generations does not. Add the same width*height check to generate_images that edit_images already has.

Closes #2586

Test Plan

pytest tests/entrypoints/openai_api/test_image_server.py

Test Result

PASS

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[david6666666]

LGTM. Thanks for your contribution.