[Core][Easy] Use envs.__getattr__ for all Unify to environment variable access

[Jialin]

Purpose

Avoid directly using TYPE_CHECKING environment variables in the code (by updating all code with from vllm.envs import FOO_BAR). And use envs.FOO_BAR instead, which go through the envs.__getattr__ which has 2 benefits:

1. no extra overhead, as environment variable caching is added in [Perf] Cache vllm.env.__getattr__ result to avoid recomputation #26146
2. using __getattr__ ensured we adopted environment variable updates during service startup time.

This should be landed after #26146

Test Plan

N/A

Test Result

N/A

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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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[gemini-code-assist]

Code Review

This pull request refactors the way environment variables from vllm.envs are accessed throughout the codebase. The changes replace direct imports of environment variables (e.g., from vllm.envs import FOO_BAR) with module-level imports (import vllm.envs as envs) and qualified access (envs.FOO_BAR). This is a good practice as it ensures that environment variable values are fetched dynamically via __getattr__, allowing for updates to be reflected during service startup. The changes are applied consistently and correctly across the modified files. The code is clean and the refactoring improves maintainability.

[Jialin]

CC @mgoin @yeqcharlotte @houseroad

[Jialin]

Resolve #26854

[mgoin]

Thanks!