fix(hyperliquid): default fetchOHLCV params to {} so server dispatch survives

Server's flat-body dispatch passes a single merged object as args[0],
leaving args[1] undefined. The other HL read methods make params
optional; OHLCV did not, so calls without explicit start/end crashed
with 'Cannot read properties of undefined (reading start)'.

Reproduces via MCP: pmxt.fetchOHLCV({exchange:'hyperliquid', outcomeId:'hl-outcome-172', resolution:'1h'}).

Author: realfishsam

core/src/exchanges/hyperliquid/index.ts

@@ -145,7 +145,7 @@ export class HyperliquidExchange extends PredictionMarketExchange {
         return this.normalizer.normalizeOrderBook(raw, outcomeId);
     }
 
-    async fetchOHLCV(outcomeId: string, params: OHLCVParams): Promise<PriceCandle[]> {
+    async fetchOHLCV(outcomeId: string, params: OHLCVParams = {}): Promise<PriceCandle[]> {
         const raw = await this.fetcher.fetchRawOHLCV(outcomeId, params);
         return this.normalizer.normalizeOHLCV(raw, params);
     }