Multi-image generation CI tests

[MohitIntel]

Test with multimodal-support for multiple images

• Current CI test for gemma3 only runs single image per prompt and its input seq_len is less than current sliding_window length (1024).
• This new test is designed such that the total input length exceeds the default sliding_window length (1024) to help validate the sliding_window mechanism is actually working or not.

[github-actions]

✅ CI Passed

All checks passed successfully against the following vllm commit:
e39dc46f8fe61803032a5f51ba76f8fa03ba0b41

[jiminha]

@xuechendi Could you review this PR? single image test doesn't cover test coverage for the sliding_window since seq_len is smaller than 1024(sliding_window size), so we are adding this.

I realized that test was added to 27b model, if you want to keep it small model(4b), we can do that as well. Let us know.

[xuechendi]

/run-gaudi-tests