[WIP] Merge PR #5865 changes into release/2.4 branch

[Copilot]

• Add FD_WORKER_ALIVE_TIMEOUT environment variable to fastdeploy/envs.py
• Update fastdeploy/entrypoints/openai/serving_chat.py to import envs and use configurable timeout
• Update fastdeploy/entrypoints/openai/serving_completion.py to import envs and use configurable timeout
• Update English documentation in docs/usage/environment_variables.md
• Update Chinese documentation in docs/zh/usage/environment_variables.md
• Add test file tests/test_worker_alive_timeout.py
• Run tests to validate changes

Original prompt

#5865 请将这个PR的diff改动合入到release/2.4分支

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