fix(data): parse combo trade activity

[cesarenaldi]

Summary

• Add public ComboTradeActivity for Data API TRADE rows with isCombo=true.
• Preserve existing CLOB TradeActivity behavior and route trade parsing on isCombo.
• Re-export ComboTradeActivity from the public package.

Linear

• DEV-270

Verification

• uv run pytest tests/unit/test_data_activity.py
• uv run ruff check .
• uv run pyright
• uv run pytest tests/unit

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Note

Low Risk
Additive parsing and typing for combo trades; standard TRADE handling is unchanged aside from an explicit is_combo flag.

Overview
Data API TRADE activity rows with isCombo: true are now parsed as ComboTradeActivity instead of failing or mis-mapping CLOB-style fields.

parse_activity branches on type == "TRADE" and isCombo before the existing type map. Combo trades use ComboConditionId, map asset to position_id, and omit CLOB-only fields (outcome, slug, event slug). Standard trades gain an explicit is_combo: false on TradeActivity.

ComboTradeActivity is part of the Activity union and is re-exported from the public package. Unit coverage was added for combo trade parsing.

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