Patch release v5.5.1 fixes Gemma4 export and vLLM CI — adds integration tests

AI Impact Summary

Patch release v5.5.1 targets two areas: Gemma4 export workflow and vLLM CI stability. The Gemma4 export fix resolves a defect that blocked exports, and the newly added integration tests (referenced as #45285) expand end-to-end coverage for Gemma4 scenarios. The vLLM CI fix (referenced as #45139) addresses CI reliability, reducing flaky builds and hidden regression risks. Collectively, teams relying on Gemma4 exports and vLLM in CI will encounter fewer blockers and faster feedback during releases.