Normalize module client base_url trailing slash

[LittleChenLiya]

Summary

Fix inconsistent base_url normalization between the module-level client and instantiated clients.

When openai.base_url is set without a trailing slash, _ModuleClient.base_url returned the raw URL, while OpenAI(base_url=...) normalized it with a trailing slash. This could lead to malformed request URLs in module-level usage.

Changes

• normalize _ModuleClient.base_url using the same trailing-slash enforcement as instantiated clients
• add a regression test for module-level base_url with a custom path and no trailing slash
• keep the module-level openai.base_url value unchanged as user input, while ensuring the resolved client URL is normalized

Testing

• python -m pytest tests/test_module_client.py -n 0
• python -m pytest tests/test_client.py -k "base_url" -n 0

Closes #1373