[Cherry-Pick][APIServer][Feature] Add configurable worker health check timeout via FD_WORKER_ALIVE_TIMEOUT(#5865)

[Copilot]

Motivation

Cherry-pick of #5865 to release/2.4 branch. Worker processes executing computations beyond 30 seconds triggered spurious "worker not healthy" errors due to hardcoded timeout in health check calls.

Modifications

Environment Variable

• Added FD_WORKER_ALIVE_TIMEOUT to fastdeploy/envs.py (default: 30s, maintains backward compatibility)

Service Layer

• Modified 4 check_health() invocations in serving_chat.py and serving_completion.py to accept configurable timeout via time_interval_threashold parameter
• Added fastdeploy.envs imports

Documentation

• Updated docs/usage/environment_variables.md and docs/zh/usage/environment_variables.md

Usage or Command

# Default 30s timeout
python -m fastdeploy.entrypoints.openai.serving_chat

# Extended timeout for long-running computations
FD_WORKER_ALIVE_TIMEOUT=120 python -m fastdeploy.entrypoints.openai.serving_chat

Accuracy Tests

N/A - Configuration change only, no model output affected.

Checklist

• Add at least a tag in the PR title.
• Format your code, run pre-commit before commit.
• Add unit tests. Please write the reason in this PR if no unit tests.
  • No unit tests added - straightforward configuration parameter with clear integration points. Original PR [APIServer][Feature] Add configurable worker health check timeout via FD_WORKER_ALIVE_TIMEOUT #5865 validated functionality.
• Provide accuracy results.
• If the current PR is submitting to the release branch, make sure the PR has been submitted to the develop branch, then cherry-pick it to the release branch with the [Cherry-Pick] PR tag.

Original prompt

#5865 请将这个PR的改动cherry pick进release 2.4分支。 需要注意，你的PR标题需要遵循规则『PR标题需遵循格式，在最开始加上[Cherry-Pick]标签，以及最后面加上原PR ID，例如[Cherry-Pick][CI] Add check trigger and logic(#5191)』

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