feat: add litellm provider which supports default api key as well as …

[tjg184]

Summary

Enables native quotas for LiteLLM.

I do see test failures still, but see same ones on main. Happy to fix, but trying to keep PR isolated to this fix.

Linked Issue

Currently using this forked version and thought it would be useful for others that use LiteLLM

OpenCode Validation

1.15.10

Quality Checklist

• I ran pnpm run typecheck
• I ran pnpm test
• I ran pnpm run build
• This is the smallest safe root-cause fix (no unnecessary hook/output mutation logic)
• I preserved behavioral invariants and updated/added boundary tests as needed
• I updated docs for user-facing workflow/command/config changes (README.md and CONTRIBUTING.md when applicable)
• For new API-key/token providers, I started from contributing/provider-template/ or explained why the template does not apply
• For provider setup/auth wording changes, I checked the relevant dummy .ts template in contributing/provider-template/ and verified README.md against src/lib/provider-metadata.ts (authentication/authFallbacks) and provider auth resolver/diagnostics behavior