Enhance task column cardinality with type groups

[Irozuku]

Summary

A task stated its column contract as one list of allowed types plus one cardinality for the whole side. That cannot express "one date column and any number of numeric ones": a single total accepts two dates and no numbers just as readily as the intended shape.

Tasks now declare a list of groups, each naming a set of interchangeable types and how many columns of that set it takes, the way BaseGenerativeTask already states its per type counts. Cardinality may be an exact int, "n", or a {min, max} range whose max may itself be "n".

metadata = {
    "inputs": [
        {"types": [Date], "cardinality": 1},
        {"types": [Float, Integer], "cardinality": {"min": 1, "max": "n"}},
    ],
    "outputs": [{"types": [Float, Integer], "cardinality": 1}],
}

The older two key spelling (inputs_types + inputs_cardinality) is read as the single group it always meant, so every existing task and plugin is unchanged. get_metadata reports both views, so every current consumer keeps reading exactly what it read before.

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Type of Change

Check all that apply like this [x]:

• Backend change
• Frontend change
• CI / Workflow change
• Build / Packaging change
• Bug fix
• Documentation

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Changes (by file)

• DashAI/back/tasks/base_task.py: added _column_groups, _bounds, _total_cardinality, _type_name, _cardinality_text and _validate_side. get_metadata now emits inputs/outputs (the per group form) alongside the four flat keys. validate_dataset_for_task delegates to the per side check, which charges each column to the first group that accepts its type and still has room, then checks every group against its own bounds. Error messages name the group's types only when there is more than one group, so single group tasks produce the message they always did.
• DashAI/front/src/components/models/modelSession/PrepareDatasetStep.jsx: added columnGroupsOf (reads metadata.inputs/outputs, falling back to the flat pair) and describeCardinality. The requirements banner renders one line per group instead of one line per side.
• DashAI/front/src/types/task.ts: added ITaskColumnGroup and the optional inputs/outputs fields on ITaskMetadataParameters.
• DashAI/front/src/utils/i18n/locales/{en,es,pt,de,zh}/experiments.json: added label.cardinalityAtLeast and label.cardinalityBetween.
• tests/back/tasks/test_task_column_groups.py: new. Covers group assignment, a group under its minimum, a group over its maximum, a type in no group, the error naming the short group, the metadata reporting both views, and the older contract still validating the way it did.
• tests/back/tasks/test_tasks.py: metadata assertions updated from 4 keys to 6, with the group shape asserted.
• tests/back/api/test_components_api.py: expected task metadata in the API responses gained the inputs/outputs keys.

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Testing

• The column picker banner on an existing task (Tabular Classification, Regression) should read exactly as before, since those declare a single group.