fix(limitless): wire unwatchOrderBook to WebSocket unsubscribe

[JSap0914]

What

LimitlessExchange never overrode unwatchOrderBook, so calling it always
threw unwatchOrderBook() is not supported by Limitless from the base class —
even though LimitlessWebSocket.unsubscribe() was already implemented.

This adds an unwatchOrderBook override that resolves the outcome slug
(mirroring the existing watchOrderBook override) and forwards it to the
websocket's unsubscribe(), which removes the subscribe_market_prices
subscription and drops the cached orderbook/price callbacks.

Why

watchOrderBook opened a subscription that could never be torn down through
the public API, so consumers had no supported way to stop streaming a market's
orderbook. The websocket already exposed unsubscribe(); this simply wires it
into the exchange interface.

The capability now reports as supported automatically via the base class's
capability derivation (exchange.has.unwatchOrderBook === true).

Tests

Added core/test/unit/limitless-unwatch-orderbook.core.test.ts:

• watchOrderBook then unwatchOrderBook forwards the resolved slug to
  LimitlessWebSocket.unsubscribe() (verified via a spy) instead of throwing.
• exchange.has.unwatchOrderBook now reports true.

# focused
npx jest -c jest.config.js test/unit/limitless-unwatch-orderbook.core.test.ts
  Tests: 2 passed, 2 total

# full core suite
npx jest -c jest.config.js
  Test Suites: 48 passed, 1 skipped, 49 total
  Tests: 812 passed, 3 skipped, 815 total

# type check
npx tsc --noEmit   # exit 0

# generators (no drift)
node core/scripts/check-exchange-drift.js   # No drift. All layers in sync.
node core/scripts/generate-compliance.js    # COMPLIANCE.md unchanged

Fixes #1660