PR3: backfill v3.18.0 fields on communications / order_groups

[TexasCoding]

Context

Backfills remaining spec v3.18.0 fields on the RFQ/Quote (communications) and OrderGroup surfaces. Closes the response-side drift on the kalshi.models.communications.* and kalshi.models.order_groups.* models.

Depends on #158. Independent of #159, #160, #PR4 — can land in parallel.

Scope

Files

• kalshi/models/communications.py — add 1 to RFQ, 3 to Quote
• kalshi/models/order_groups.py — add 1 to OrderGroup, 1 to GetOrderGroupResponse, 2 to CreateOrderGroupResponse
• Tests: extend the existing communications + order_groups test suites

Field-level changes

communications.RFQ (+1):

Field	Spec type	SDK type	Notes
creator_subaccount	integer	int | None = None	subaccount index of the RFQ creator

communications.Quote (+3):

Field	Spec type	SDK type	Notes
creator_subaccount	integer	int | None = None	quote creator's subaccount
post_only	boolean	bool | None = None	mirrors CreateQuoteRequest.post_only (already on the request side from v2.1.0)
rfq_creator_subaccount	integer	int | None = None	RFQ creator's subaccount, surfaced on the quote

order_groups.OrderGroup (+1):

Field	Spec type	SDK type	Notes
exchange_index	integer	int | None = None	mirrors Market/Order/Event

order_groups.GetOrderGroupResponse (+1):

Field	Spec type	SDK type	Notes
exchange_index	integer	int | None = None

order_groups.CreateOrderGroupResponse (+2):

Field	Spec type	SDK type	Notes
exchange_index	integer	int | None = None
subaccount	integer	int | None = None	server echoes the subaccount the group was created against

Conventions

Same as #159, #160.

Acceptance criteria

• These five parameterized cases pass without warnings:
  • tests/test_contracts.py::TestSpecDrift::test_additive_drift[RFQ]
  • tests/test_contracts.py::TestSpecDrift::test_additive_drift[Quote]
  • tests/test_contracts.py::TestSpecDrift::test_additive_drift[OrderGroup]
  • tests/test_contracts.py::TestSpecDrift::test_additive_drift[GetOrderGroupResponse]
  • tests/test_contracts.py::TestSpecDrift::test_additive_drift[CreateOrderGroupResponse]
• 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

• Quote.post_only is a server echo of the value passed on CreateQuoteRequest — verify in the spec that it returns on Quote (not just on CreateQuoteResponse). Should mirror.
• CreateOrderGroupResponse.subaccount is distinct from CreateOrderGroupResponse.exchange_index. Both are surfaced together because the server echoes the routing context.

Effort

~30 lines model code + ~15 lines tests.

Part of the response-side spec drift hardening stack.

---

Tracking: #157.