fix: correct order snapshot subscription types

[harley-poly]

Changes

Allow SUBSCRIPTION_TYPE_ORDER_SNAPSHOT in private subscriptions and correct the OrderSnapshot discriminator. Add a consumer type fixture to the existing mypy job, cover request types and successful/failed EOF dispatch, and update the README to request snapshots separately from live orders.

Runtime dispatch stays the same. Consumers relying on the previous incorrect snapshot literal may need to update their type annotations. README code-block formatting also satisfies the current Ruff check.

Testing

• uv run pytest — 131 tests pass.
• uv run ruff check . and uv run ruff format --check . pass.
• uv run mypy polymarket_us tests/types/order_snapshot.py — 32 files pass; the consumer fixture produces two expected errors against the original types.
• uv build passes.

Checklist

• Tests pass (pytest)
• Linting passes (ruff check .)
• Types check (mypy polymarket_us)

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Note

Low Risk
Runtime WebSocket handling is unchanged; the main impact is stricter typings that may require consumer annotation updates.

Overview
Fixes WebSocket typing and docs so one-shot order snapshots use SUBSCRIPTION_TYPE_ORDER_SNAPSHOT, separate from live SUBSCRIPTION_TYPE_ORDER updates. OrderSnapshot’s subscriptionType literal is corrected accordingly, and PrivateSubscriptionType now allows subscribing with the snapshot type.

CI mypy also type-checks tests/types/order_snapshot.py as a consumer fixture. New offline tests assert live vs snapshot subscribe payloads differ and that terminal snapshot frames dispatch to order_snapshot or error when an error field is present. The README explains requesting snapshots separately, shows an extra subscribe call, and prints eof in the snapshot handler (plus minor example formatting).

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[poly-william]

Reviewed this exact head. Snapshot typing matches the gateway contract, runtime dispatch is unchanged, and local tests, typing, lint and build passed. Current CI is green.