Fix NameError when pydot is not installed in Beam Playground

[liferoad]

What

When pydot is not installed, the import in pipeline_graph.py fails silently (caught by except ImportError: pass), and subsequent calls to pydot.Dot() raise a confusing NameError with no context about what is missing or how to fix it.

How

_construct_graph() now checks if pydot is available before use and raises a clear RuntimeError with install instructions instead.

Testing

Manually verified the fix works by importing the module with and without pydot installed.

Fixes #37829

[gemini-code-assist]

Summary of Changes

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This pull request improves the error handling in the Beam Playground's pipeline graph generation. By explicitly checking for the presence of the pydot dependency, it prevents confusing runtime errors and provides actionable feedback to users when the library is missing.

Highlights

• Error Handling Improvement: Added a check to verify pydot availability before graph construction.
• User Feedback: Replaced a cryptic NameError with a clear RuntimeError providing installation instructions.

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