feat: add websocket auto-reconnect, resubscribe, and re-auth

[harley-poly]

summary

makes the private and markets websockets resilient to dropped connections:

• automatic reconnect with exponential backoff + jitter on unexpected drops (auto_reconnect=True by default; reconnect_max_attempts configurable).
• each reconnect re-signs the auth handshake with a fresh timestamp/signature (the api authenticates only at upgrade), and replays every active subscription.
• subscriptions are tracked on subscribe/unsubscribe so they can be replayed; a reconnect event fires after a successful reconnect.
• reconnect stops on fatal upgrade failures (401/403/429); transient/network failures keep retrying. supports both modern and legacy websockets exception shapes.

note: per the server protocol, order/position/trade streams don't replay history on resubscribe (market-data and account-balance snapshots are re-sent automatically); use SUBSCRIPTION_TYPE_ORDER_SNAPSHOT to refetch open orders after a reconnect.

test plan

• ruff check / ruff format --check
• mypy polymarket_us
• pytest (127 passed; new tests/test_websocket_reconnect.py covers subscription tracking, resubscribe replay, transient-then-success reconnect, fatal-auth stop, max-attempts, and the upgrade-status helper)

---

Note

Medium Risk
Changes live streaming connection semantics for authenticated trading and market feeds; missed or stale order/position state is possible if callers ignore the documented non-replay behavior after reconnect.

Overview
WebSocket clients (private and markets) now recover from unexpected disconnects by default: exponential backoff with jitter, a fresh signed upgrade on each attempt, and automatic replay of every subscription still registered via subscribe (cleared on unsubscribe). Successful recovery emits a reconnect event; auto_reconnect and reconnect_max_attempts tune behavior, and upgrade failures 401/403/429 stop retries instead of looping.

The README documents that order / position / trade streams do not replay historical updates after reconnect (snapshots for market data and balances are described as automatic; open orders may need an order snapshot subscription).

Supporting changes: close() cancels the background run/reconnect task; socket cleanup before retry avoids leaks; MarketsWebSocket / PrivateWebSocket accept broader constructor kwargs for reconnect options; auth test fix for 64-byte keys; new tests/test_websocket_reconnect.py coverage.

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