Fix OpenAI model discovery base URL override

[Genmin]

Summary

• use the configured OpenAI base_url for model discovery instead of always calling https://api.openai.com/v1/models
• avoid appending /models twice when an OpenAI-compatible endpoint already includes that path
• add regression coverage for OpenAI and OpenAI-compatible discovery URLs

Validation

• uv run pytest tests/test_credentials_api.py -q
• uv run ruff check api/credentials_service.py tests/test_credentials_api.py

Fixes #676

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.