Function validate_plugin_configs 

pub fn validate_plugin_configs(
    steps: &[StepDef],
    registry: &PluginRegistry,
) -> Vec<String>

Validate plugin step configurations against each plugin’s declared schema.

For each step whose type matches a registered plugin:

• Ensures all required_fields are present in the step config
• Reports missing required fields as errors
• Applies default values for missing optional fields (mutates the steps)

Returns a list of validation error messages (empty means all ok).