[mypy] Enable type checking for more directories

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Purpose

Run full mypy check and make them pass against the following directories:

• vllm/engine (except arg_utils.py)
• vllm/plugins
• vllm/worker
• vllm/v1/metrics
• vllm/v1/sample

Test Plan

Test Result

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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 in the Google Doc.

[gemini-code-assist]

Code Review

This pull request enables mypy type checking for several new directories (vllm/engine, vllm/plugins, vllm/worker, vllm/v1/sample) by updating the pre-commit configuration. The associated changes are fixes to type annotations to make the code compliant with the stricter checks, such as allowing None for certain parameters and using TYPE_CHECKING for optional imports. The changes are correct and improve the overall type safety of the codebase. I have no further comments.

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Pre-commit passes now

[DarkLight1337]

Flaky test