fix(CFT-223): Handle empty list to dict conversion for user_properties

[davidesner]

Summary

Fixes the "eternal KBC bug" (CFT-223) where the Keboola platform converts empty JSON objects {} to empty arrays [] in configuration parameters. When users leave user_properties empty, the platform preprocessing converts it from {} to [], but the Configuration dataclass expected a dictionary, causing dacite validation to throw a WrongTypeError.

Changes:

• Updated user_properties type hint to accept both dict[str, object] | list so dacite can deserialize either type
• Added __post_init__ method to Configuration class that converts empty lists to empty dicts
• Non-empty lists raise UserException with a clear error message
• Added 5 unit tests covering: empty list conversion, non-empty list rejection, dict unchanged, empty dict unchanged, and default value

Review & Testing Checklist for Human

• Verify dacite integration: The unit tests only test the Configuration dataclass directly. Verify that when config.json contains "user_properties": [], dacite.from_dict() in component.py correctly passes the list to the dataclass and __post_init__ converts it to {}.
• Test in Keboola environment: Create a configuration with empty user_properties in the UI and verify the component runs without the WrongTypeError.
• Verify downstream code: Confirm that _merge_user_parameters() (line 119-131 in component.py) receives a dict after the conversion.

Recommended test plan:

1. Deploy this branch to a test environment using runtime.tag configuration
2. Create a Custom Python component configuration with empty user_properties
3. Run the component and verify it initializes successfully
4. Verify user scripts can access parameters via CommonInterface

Notes

• This is defensive programming for known platform behavior - the platform team has confirmed this {} to [] conversion is expected
• The fix is localized to the Configuration dataclass layer using __post_init__ as requested

Link to Devin run: https://app.devin.ai/sessions/0dcae42daa1342af93cdf52c06e42d6c
Requested by: david@keboola.com (@davidesner)

[linear]

CFT-223 Custom Python - handle the eternal KBC bug

[devin-ai-integration]

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[soustruh]

LGTM 👍