feat: Add API key validation utility to prevent runtime crashes

[sachdevlaksh]

Summary

This prevents runtime errors when users attempt to use providers without proper API key setup.

• Add validate_provider_config() function to check required environment variables
• Enhance resolve_provider() to validate and warn about missing API keys
• Update doctor command to include provider configuration check
• Support validation for all providers: anthropic, openai, gemini, ollama, litellm
• Prevent crashes by validating configuration before provider instantiation

Key Point
User steps remain unchanged.
New behavior: proactive configuration validation and warnings.
No extra CLI flags or modes required

Related Issues

Test Plan

• Tests pass (pytest)
• Lint passes (ruff check .)
• 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

[RaghavChamadiya]

Great work here: validate_provider_config() and the doctor integration are genuinely useful additions that go beyond what the other PRs touch. We're merging #21 for the provider constructor fix, but your helpers.py changes (passing api_key explicitly in resolve_provider) and the doctor check are worth landing separately. Could you rebase on main after #21 merges and drop the anthropic.py/openai.py changes (they'll already be in)? Happy to merge the rest.

[sachdevlaksh]

Rebased onto main, dropped the provider-file changes already merged in #21, and kept only the helpers/doctor/test updates. The branch is now squashed to one commit on top of main. Could you please take another look?

[RaghavChamadiya]

lgtm