samples: DaoXE OpenAI-compatible gateway via LiteLlm

[seven7763]

Summary

Adds a contributing sample showing how to run an ADK agent against an OpenAI-compatible multi-model gateway using the existing `LiteLlm` wrapper.

• `contributing/samples/models/hello_world_daoxe/` (agent + main + README)
• Env: `OPENAI_API_KEY` + `OPENAI_API_BASE=https://daoxe.com/v1\`
• Model: `LiteLlm(model="openai/")`

Same pattern as the Ollama README’s “Using openai provider” path.

Notes

• Account-scoped model IDs (no static catalog)
• Chat Completions via LiteLLM OpenAI provider
• DaoXE is not available in mainland China
• Contributor is affiliated with DaoXE

Test plan

• Sample imports cleanly
• Optional: `adk web` / main with real DaoXE key + model ID

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[seven7763]

Google CLA: I will complete signing at https://cla.developers.google.com/ if not already on file for this GitHub account.

[rohityan]

Hi @seven7763 , Thank you for your contribution! It appears you haven't yet signed the Contributor License Agreement (CLA). Please visit https://cla.developers.google.com/ to complete the signing process. Once the CLA is signed, we'll be able to proceed with the review of your PR. Thank you!