docs: describe long-running tool progress options

[adityasingh2400]

Summary

• Document the current limitation that function tools return one final output and do not stream custom intermediate tool-progress events into Runner.run_streamed().
• Point users to current options: nested agent on_stream, app-owned progress channels, or durable workflow integrations.

Test plan

• uv run mkdocs build
• git diff --check

Related to #1333.

[seratch]

Thinking of sandbox agent direction, this is not the thing we'd like to encourage.