docs/examples: add a minimal Qianfan onboarding example

[jimmyzhuu]

Summary

This PR adds a small, built-in OpenAI-compatible onboarding path for Baidu Qianfan without introducing any new provider abstraction.

Changes in scope:

• adds 1 new example: examples/model_providers/qianfan_provider.py
• adds 1 focused test module: tests/test_qianfan_model_provider_example.py
• adds 1 example discovery assertion in tests/test_run_examples_script.py
• updates 2 existing documentation entry points:
  • examples/model_providers/README.md
  • docs/models/index.md

What the example does:

• configures AsyncOpenAI against Qianfan's OpenAI-compatible base URL
• uses set_default_openai_client(...)
• uses the Chat Completions path via set_default_openai_api("chat_completions")
• demonstrates one minimal function tool call
• exits safely with a skip message when QIANFAN_API_KEY is not set

Why this shape:

• it stays within the repository's existing "non-OpenAI models" integration patterns
• it keeps the first Qianfan-facing change reviewable and low-risk
• it avoids introducing a vendor-specific abstraction for a single provider example

Validation notes:

• the Qianfan onboarding path in this PR was validated locally against ernie-5.0
• baseline chat and function tool calling both succeeded on the OpenAI-compatible Chat Completions path
• the example and tests remain CI-safe because automated coverage does not require external credentials
• this PR focuses on the provider path that was validated end-to-end for onboarding

Non-goals:

• no LiteLLM or any-llm routing changes
• no top-level README changes
• no translation updates
• no network-dependent test coverage in CI

Test plan

Automated local verification:

uv run pytest tests/test_qianfan_model_provider_example.py tests/test_run_examples_script.py::test_discover_examples_finds_qianfan_provider -v

Result:

• 3 targeted checks passed
• verified default Qianfan settings loading
• verified the no-credential safe-exit path
• verified that the new example is discoverable by the example runner

Credentialed provider validation against Qianfan's OpenAI-compatible endpoint:

• listed accessible models for the provided account and confirmed the validated model ID is ernie-5.0
• ran a live smoke test against ernie-5.0
• verified baseline chat succeeds
• verified function tool calling succeeds

Example safety check without credentials:

uv run python -m examples.model_providers.qianfan_provider

Result:

• prints Skipping run because no valid QIANFAN_API_KEY was provided.
• exits successfully without attempting a provider call

Formatting and linting:

make format
make lint

Result:

• formatting completed successfully
• lint checks passed

Docs verification:

make build-docs

Result:

• docs build completed successfully
• no docs errors were introduced by the Qianfan example/docs changes

Issue number

N/A

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[seratch]

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