fix: preserve WebSocket error subscription context

[harley-poly]

Changes

Server error frames include subscriptionType, but the SDK drops it when constructing the callback error. Preserve it as optional WebSocketError.subscription_type on both streams and add the field to WebSocketErrorMessage. Existing error text, request IDs and dispatch behavior stay unchanged.

Document the optional context and set the patch version to 2.4.1.

Part of ISVBE-939. SDK follow-up to https://github.com/Polymarket/go-gateway-us/pull/2267; the ticket's broader marshaler work remains separate.

Testing

• 288 tests pass on Python 3.13/websockets 17.2 and Python 3.10/websockets 13.0.
• Local socket regressions cover both streams, known/future/unspecified types, absent context and raw messages. Removing the forwarding fails 12 cases; the four context-free cases still pass.
• Ruff lint/format, mypy with typed consumers, and wheel/sdist build pass.

Checklist

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

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Note

Low Risk
Backward-compatible additive metadata on WebSocket error handling; no changes to auth, trading APIs, or connection lifecycle.

Overview
WebSocket server error frames can include subscriptionType, but the SDK previously dropped it when building WebSocketError for the error callback. This change forwards that field as an optional subscription_type attribute (alongside existing request_id) on both markets and private streams, and extends WebSocketErrorMessage typing accordingly.

Error message text, callback dispatch, and repr behavior stay the same when the server omits context. README notes the new optional fields; release is bumped to 2.4.1 with regression tests across streams and subscription-type variants.

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