Fix inconsistent base_url behavior in module client and OpenAIClient

[SmartManoj]

Fixes #1373

Update base_url behavior to enforce trailing slash in module client

• Modify src/openai/__init__.py to enforce a trailing slash for base_url in the _ModuleClient class.
• Add a helper method _enforce_trailing_slash to the _ModuleClient class.
• Update tests in tests/test_module_client.py to verify the consistent behavior of base_url in both module client and OpenAIClient.

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