fix(gamma): preserve market group item title

[cesarenaldi]

Summary

• add group_item_title to the Market model
• preserve groupItemTitle when parsing market responses
• cover the field in the existing gamma model test

Verification

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

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Note

Low Risk
Optional string field on a Pydantic model with a unit test; no auth, trading, or behavioral logic changes.

Overview
Gamma Market parsing now keeps groupItemTitle from API payloads as group_item_title, matching how grouped markets label individual legs.

The new field is on the model (with a groupItemTitle alias) and is copied in _normalize_market when flattening raw responses. A unit test asserts the value survives Market.parse_response.

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