[Frontend] Add video decode cache

[DesertsP]

Summary

• Add an in-process LRU cache for decoded video files in VideoMediaIO.load_file.
• Key cache entries by resolved path, mtime, size, requested frame count, backend, and loader kwargs.
• Deduplicate concurrent decodes for the same file and return defensive copies of cached frames/metadata.
• Add VLLM_VIDEO_DECODE_CACHE_SIZE to control cache size. The default is 0, so the cache is disabled unless users opt in with a positive size.

Duplicate-work check

• Checked open PRs with gh pr list --repo vllm-project/vllm --state open --search "video decode cache".
• Checked open PRs with gh pr list --repo vllm-project/vllm --state open --search "video cache".
• Checked open PRs with gh pr list --repo vllm-project/vllm --state open --search "multimodal video".
• Checked open issues with gh issue list --repo vllm-project/vllm --state open --search "video decode cache".
• No open PR was found that directly implements decoded video file caching. Related but different work includes [Multimodal] Optimize sparse frames decoding for PyAV video backend #44598 for sparse PyAV frame decoding optimization and [Profiler] Get Detailed MultiModal Data Preprocessing Timing Stats #44404 for multimodal preprocessing timing stats.

Tests

• git diff --check origin/main..HEAD passed.
• .venv/bin/python -m py_compile vllm/multimodal/media/video.py vllm/envs.py tests/multimodal/media/test_video.py passed.
• python -m pytest tests/multimodal/media/test_video.py -v -s passed: 15 passed, 13 warnings.

AI assistance

• AI assistance was used to prepare this PR branch, resolve rebase/cherry-pick conflicts against origin/main, run checks, fix the cache-size handling found by tests, and draft this PR description. The human submitter must review every changed line and be able to defend the change end-to-end.

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[mergify]

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

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

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[mergify]

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

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

[mergify]

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

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

[DarkLight1337]

Thanks for the PR! We are already making similar efforts to avoid redundant media loading, see #55583 and #57752