[Multimodal] Add Qwen2-VL/Qwen2.5-VL processor-mapped video loader

[WindChimeRan]

Purpose

Qwen2-VL and Qwen2.5-VL currently fall back to the generic opencv video loader (uniform num_frames=32) because they ship no video_processor_type in their preprocessor config.

This adds Qwen2VLVideoBackend, so the loader is auto-selected (#44126). Same pattern as the Qwen3-VL loader (#44412). This will be the baseline groundtruth of #45203

fps takes precedence over num_frames because vLLM's VideoMediaIO always injects num_frames=32, which would otherwise make fps sampling unreachable; default fps=2.

Result

Qwen2.5-VL-7B-Instruct, NExTQA (100) + MVBench (90), offline llm.chat, max_tokens=1, 1×A100, identical engine config (max_pixels=128·28²), back-to-back:

loader	NExTQA	MVBench	overall	req/s
opencv num_frames=32 (main default)	0.800	0.611	0.711	2.16
opencv fps=2, cap 768 (same-budget control)	0.840	0.700	0.774	1.45
qwen2_vl fps=2 (this PR)	0.840	0.700	0.774	1.57

Correct sampling gains accuracy (+8.9 pt on MVBench's motion/temporal tasks) at a throughput cost (fps=2 samples more frames than a fixed 32).

Test Plan / Result

Extends test_video_processor_from_model_repo with both repos (exercises the model_type resolver fallback) and adds test_qwen2vl_loader_matches_hf_sample_frames, which asserts the loader's indices equal Qwen2VLVideoProcessor.sample_frames` across an fps/duration grid (including an out-of-range-tail case that exercises the clamp).

pytest tests/multimodal/test_video.py -k qwen2vl -v   # 4 passed
pytest tests/multimodal/test_video.py -q              # passed, no regressions

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[WindChimeRan]

@Isotr0py followup of #45203

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @WindChimeRan.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[WindChimeRan]

fixed CI tests.

[Isotr0py]

Seems https://buildkite.com/vllm/ci/builds/73265#019ee942-329b-4f80-bfe2-37f276a6d480 failure also related.

[2026-06-21T09:16:35Z] FAILED entrypoints/pooling/classify/test_online_vision.py::test_chat_video_url_request[muziyongshixin/Qwen2.5-VL-7B-for-VideoCls] - AssertionError: assert 8993 == 4807
[2026-06-21T09:16:35Z]  +  where 8993 = UsageInfo(prompt_tokens=8993, total_tokens=8993, completion_tokens=0, prompt_tokens_details=None).prompt_tokens
[2026-06-21T09:16:35Z]  +    where UsageInfo(prompt_tokens=8993, total_tokens=8993, completion_tokens=0, prompt_tokens_details=None) = ClassificationResponse(id='classify-a68fd2ee63ce0dfd', object='list', created=1782032047, model='muziyongshixin/Qwen2.5-VL-7B-for-VideoCls', data=[ClassificationData(index=0, label='LABEL_1', probs=[3.55368274540524e-06, 0.9999964237213135], num_classes=2)], usage=UsageInfo(prompt_tokens=8993, total_tokens=8993, completion_tokens=0, prompt_tokens_details=None)).usage