fix(models): expose missing activity market icons as None

[cesarenaldi]

Sparse historical activity rows come back with an empty-string icon. The strict activity models (TradeActivity and the split/merge/redeem/conversion variants) currently surface that "" as-is, while the TypeScript SDK exposes missing icons as null.

This aligns the contract: icon is now str | None on the strict activity models, and empty-string icons are normalized to None during activity payload normalization (same sentinel handling already used for conditionId/asset/side/outcome).

Note: the API serializes icon as a string and never emits null (verified against the payload source and live responses), so this is a parity/ergonomics fix rather than a parse-failure fix.

Verification

• Added unit tests: TRADE and SPLIT payloads with icon: "" parse with icon is None.
• uv run pytest tests/unit -q: 1698 passed.
• uv run ruff check ., uv run ruff format --check .: clean.
• uv run pyright: 0 errors.
• Live smoke: parsed 500 activity rows from a wallet with 129 sparse rows; all sparse icons surface as None, populated icons unchanged.

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Note

Low Risk
Small model and normalization change for an optional display field; no auth, payments, or persistence logic.

Overview
Activity icon parity with the TypeScript SDK: sparse API rows send icon as "". TradeActivity and market-event activities (Split, Merge, Redeem, Conversion) now type icon as str | None (default None) instead of required str.

Normalization: _normalize_activity_payload treats empty icon like other sentinels (conditionId, asset, etc.)—the key is dropped so parsing yields None rather than "".

Unit tests cover TRADE and SPLIT payloads with icon: "".

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