[POC] Support Python workflow args in CLI

[oliverholworthy]

Summary

POC for #507 that makes local Python config workflows executable with custom CLI arguments while keeping the existing preview, create, and validate commands as the execution surface.

This lets a workflow file expose:

def load_config_builder(params: dd.DataDesignerScriptParams) -> dd.DataDesignerConfigBuilder:
    ...

and run with:

data-designer preview workflow.py -- --custom-arg value
data-designer create workflow.py -- --custom-arg value
data-designer validate workflow.py -- --custom-arg value

Design

• Adds DataDesignerScriptParams as the small runtime object passed to Python config modules.
• Preserves the existing no-argument load_config_builder() contract for current workflow files.
• Forwards arguments after -- only to local Python config modules.
• Rejects forwarded workflow arguments for static YAML, JSON, and URL configs because those config sources do not execute argument-parsing code.
• Treats argparse --help from a workflow as a successful CLI exit instead of a config load failure.

Why This PR Is Separate

This is the mergeable base layer. It is independently useful for executable workflow files and gives recipe discovery a clean primitive to build on later.

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