[Fix] make images LoRA e2e less flaky

[dongbo910220]

Purpose

Fix #977

• Fix flaky tests/e2e/online_serving/test_images_generations_lora.py::test_images_generations_per_request_lora_switching in
  CI.
• Increase the synthetic LoRA perturbation strength so the LoRA effect is consistently larger than the baseline drift after
  switching back to no-LoRA.
• No production logic changes (test-only change).

Test Plan

• pytest -s -v tests/e2e/online_serving/test_images_generations_lora.py

Test Result

• passed

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft.

BEFORE SUBMITTING, PLEASE READ https://github.com/vllm-project/vllm-omni/blob/main/CONTRIBUTING.md (anything written below this line will be removed by GitHub Actions)

[dongbo910220]

@david6666666

[david6666666]

LGTM

[david6666666]

You should add "Diffusion Images API LoRA E2E" test ,refer to #975

[dongbo910220]

You should add "Diffusion Images API LoRA E2E" test ,refer to #975

@david6666666 added and CI passed

[david6666666]

Please fix CI

[dongbo910220]

Please fix CI

@david6666666 The lora CI test passed and CI failure is about test_qwen3_omni.py which is not related with the PR, pls retry it.

[hsliuustc0106]

Please fix CI

@david6666666 The lora CI test passed and CI failure is about test_qwen3_omni.py which is not related with the PR, pls retry it.

this is a know issue after #956 merged, but it does not occur every time

[dongbo910220]

this is a know issue after #956 merged, but it does not occur every time

Got it. Hope this time passed!

[hsliuustc0106]

lgtm