docs(agent-tools): add example for custom model provider propagation (#663)

[furkankoykiran]

What this PR does / why we need it

This PR adds an example demonstrating the behavior fixed in #2463 where a custom ModelProvider specified via RunConfig is correctly inherited when using an agent as a tool.

Since the core engineering fix to Agent.as_tool() was already merged in #2463, this documentation addition serves to close out the remaining gap from #663 by giving users a runnable example of this requested pattern.

Changes

• Adds examples/agent_patterns/agents_as_tools_custom_provider.py which shows how a custom ModelProvider is passed to the orchestrator agent and properly inherited by nested agent-tools.
• Updates examples/agent_patterns/README.md to reference the new example.

Fixes #663

[seratch]

Thanks for sending this patch! However, we already have a similar example at https://github.com/openai/openai-agents-python/blob/main/examples/model_providers/custom_example_provider.py, so we won't have this one.