PR2: backfill v3.18.0 fields on events / portfolio / historical / incentive_programs

[TexasCoding]

Context

OpenAPI v3.18.0 added fields to several mid-tier response models. None individually justify a dedicated PR, but grouping them keeps each model surface coherent. Removes 9 of the remaining additive-drift warnings.

Depends on #158. Independent of #159, #PR3, #PR4 — can land in parallel.

Scope

Files

• kalshi/models/events.py — add 3 to Event, 2 to EventMetadata
• kalshi/models/portfolio.py — add 1 to Settlement
• kalshi/models/historical.py — add 2 to Trade
• kalshi/models/incentive_programs.py — add 1 to IncentiveProgram
• Tests: extend the relevant tests/test_<resource>.py to cover (de)serialization of new fields

Field-level changes

Event (+3):

Field	Spec type	SDK type	Notes
exchange_index	integer	int | None = None	matches the same field on Market/Order
fee_multiplier_override	number	DollarDecimal | None = None	confirm — spec may type as plain number or string; check at edit time
fee_type_override	string	str | None = None	enum-like; keep str

EventMetadata (+2):

Field	Spec type	SDK type	Notes
competition	string	str | None = None	sport/event competition name
competition_scope	string	str | None = None

Settlement (+1):

Field	Spec type	SDK type	Notes
value	integer	int | None = None	payout per yes contract, in cents per spec description. Plain int, not DollarDecimal — verify at edit time

Trade (+2):

Field	Spec type	SDK type	Notes
taker_outcome_side	string	SideLiteral | None = None	Literal["yes", "no"] — mirrors Order.outcome_side in #159
taker_book_side	string	BookSideLiteral | None = None	Literal["bid", "ask"]

IncentiveProgram (+1):

Field	Spec type	SDK type	Notes
incentive_description	string	str | None = None	optional descriptive blurb returned alongside the program

Conventions

Same as #159:

• Append-only at end of each model.
• Use existing *Literal aliases (SideLiteral, BookSideLiteral from kalshi/models/orders.py — import if needed).
• extra="allow" policy already set on each model; do not change.

Acceptance criteria

• uv run pytest "tests/test_contracts.py::TestSpecDrift::test_additive_drift[Event]" "tests/test_contracts.py::TestSpecDrift::test_additive_drift[EventMetadata]" "tests/test_contracts.py::TestSpecDrift::test_additive_drift[Settlement]" "tests/test_contracts.py::TestSpecDrift::test_additive_drift[Trade]" "tests/test_contracts.py::TestSpecDrift::test_additive_drift[IncentiveProgram]" -v — all five pass without warnings.
• uv run pytest tests/ -q --ignore=tests/integration passes.
• uv run mypy kalshi/ strict-clean.
• uv run ruff check . clean.
• At least one (de)serialization test per backfilled model.

Notes

• Settlement.value description from spec: "Payout of a single yes contract in cents." → int, not DollarDecimal. Confirm by reading specs/openapi.yaml Settlement.value.
• Cross-model field naming: exchange_index appears on Market (PR1), Order (PR1), Event (this PR), OrderGroup/{Get,Create}OrderGroupResponse (PR3). All int | None = None. Keep the same shape.

Effort

~50 lines model code + ~20 lines tests.

Part of the response-side spec drift hardening stack.

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Tracking: #157.