[FIX] fixing bug in using AsyncOpenAI client (header setting, token counting, etc)

[gangmuk]

Pull Request Description

This PR fixes a few bugs in client and also improves failure request handling. Bugs

• Header setting in AsyncOpenAI was not being done (routing-strategy header)
• It was counting the number of output tokens in a wrong way in streaming client.

Improvement

• Handling failure request in a better way

Related Issues

Resolves: #[Insert issue number(s)]

Important: Before submitting, please complete the description above and review the checklist below.

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Your PR title should start with one of these prefixes to indicate the nature of the change:

• [Bug]: Corrections to existing functionality
• [CI]: Changes to build process or CI pipeline
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Submission Checklist

• [ ] PR title includes appropriate prefix(es)
• [ ] Changes are clearly explained in the PR description
• [ ] New and existing tests pass successfully
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• [ ] Documentation updated to reflect changes (if applicable)
• [ ] Thorough testing completed, no regressions introduced

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[Jeffwan]

For the CI issue, I left comment here https://github.com/vllm-project/aibrix/issues/739 and please take a look