fix(bonus-unit2): update deprecated Langfuse API and add missing ddgs dependency

[meatballg1210]

Problem

1. The "Additional Attributes" section uses deprecated Langfuse Python SDK v3 APIs
   that have been removed in v4:

   • start_as_current_span() → replaced by start_as_current_observation()
   • span.update_trace() → replaced by propagate_attributes()

2. DuckDuckGoSearchTool requires the ddgs package which is not included
   in the install command. It belongs to smolagents' toolkit optional group,
   not telemetry, so smolagents[telemetry] does not install it.

Changes

1. Updated "Additional Attributes" code: start_as_current_span →
   start_as_current_observation, update_trace → propagate_attributes
2. Added ddgs to the pip install command in Step 0

Verification

• Tested in a clean Google Colab environment
• Without ddgs: DuckDuckGoSearchTool() fails with ModuleNotFoundError
• With ddgs added: works correctly
• Confirmed new API matches Langfuse SDK v4 documentation

References

• Langfuse v3→v4 migration: https://langfuse.com/docs/observability/sdk/upgrade-path/python-v3-to-v4
• Langfuse instrumentation docs: https://langfuse.com/docs/observability/sdk/instrumentation
• smolagents pyproject.toml: https://github.com/huggingface/smolagents/blob/main/pyproject.toml

[meatballg1210]

Hi maintainers 👋 There's still 1 workflow awaiting approval. It would be great if someone could approve it and review the changes when you have a moment. Happy to address any feedback. Thanks!