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

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