fix: correct allMids lookup key in hyperliquid normalizer

[realfishsam]

Fixes #441

[realfishsam]

PR Review: VERIFIED

What This Does

Fixes the allMids lookup key used in the Hyperliquid normalizer and fetcher. The allMids API endpoint keys prediction-market outcomes as @{outcomeId} (e.g. @8), not the #{encoding} coin notation used for orders/positions. This was causing mid-price lookups to silently miss, defaulting all prices to 0.5.

Blast Radius

• core/src/exchanges/hyperliquid/fetcher.ts -- getMidForOutcome() now uses toMidKey()
• core/src/exchanges/hyperliquid/normalizer.ts -- normalizeEventWithMarkets() now uses toMidKey()
• core/src/exchanges/hyperliquid/utils.ts -- new toMidKey() utility added

No SDK or OpenAPI changes needed since this is a core-internal data lookup fix.

Findings

No blocking findings. The change is clean: a new pure function is introduced, imports are updated, and both call sites (fetcher + normalizer) are updated consistently.

PMXT Pipeline Check

• Field propagation: N/A (no new fields)
• OpenAPI sync: N/A (no API surface change)
• Type safety: OK

Semver Impact

patch -- bug fix for incorrect price lookups on Hyperliquid

Risk

The @{outcomeId} key format is not documented in a spec; it was inferred from the live API. If Hyperliquid changes this format, the lookup will silently miss again and fall back to 0.5. No test covers the toMidKey mapping against a real or mocked allMids response.