[CI] Set L2 & L3 tests running conditions.

[congw729]

PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.

Purpose

Set L2 & L3 tests running conditions.

Now:
pipeline.yml -> the entry point for setting up which tests should run in this CI.
When ready is labeled -> execute test-ready.yaml (old tests in pipeline.yml)
When PP is merged to the main branch -> execute test-merge.yaml
Nightly -> execute test-nightly.yaml

Fix:
Remove slow tests - Qwen3omni offline cases.
Fix L3 Qwen3omni test case.

Test Plan

Upload to buildkite and check.

Test Result

---

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. Please run mkdocs serve to sync the documentation editions to ./docs.
• (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)

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[congw729]

@david6666666 Please add a ready label.

[SamitHuang]

@congw729 please resolve the conflicts