refactor(ollama): update base URL handling to automatically append /v1 suffix for OpenAI SDK compatibility

[gtkacz]

Summary

• The provider owns the contract with the OpenAI SDK. It should handle the URL format it needs, not push that responsibility onto every caller (CLI, server, tests, future integrations).
• _normalize_base_url is safe to call on URLs that already have /v1 as it won't double-append. This means callers who were already passing /v1 (if any) won't break.
• OLLAMA_BASE_URL=http://localhost:11434 is the natural, documented convention that matches Ollama's own docs. The /v1 is an implementation detail of the OpenAI compatibility layer.

Related Issues

Closes #30

Test Plan

• Tests pass (pytest)
• Lint passes (ruff check .) (Has errors but not from my changes)
• Web build passes (npm run build) (if frontend changes)

Checklist

• My code follows the project's code style
• I have added tests for new functionality
• All existing tests still pass
• I have updated documentation if needed

[swati510]

nit: PR title says "OpenAPI" but should be "OpenAI"