fix: read eventId instead of questionId in myriad normalizer

[realfishsam]

Fixes #556

[realfishsam]

PR Review: FAIL

What This Does

Renames questionId to eventId across three Myriad files to match the current live API response shape. This is v2 of PR #586, which missed utils.ts:68.

The three changes:

1. fetcher.ts:20 -- MyriadRawMarket interface field renamed from questionId to eventId
2. normalizer.ts:42 -- normalizeMarket() now reads raw.eventId instead of raw.questionId
3. utils.ts:68 -- mapMarketToUnified() now reads market.eventId instead of market.questionId (the line v1 missed)

Blast Radius

• Contained to core/src/exchanges/myriad/ -- three files, one line each.
• No other exchanges are affected. The questionId references in metaculus/, hyperliquid/, and opinion/ are venue-specific and unrelated.
• The MyriadRawMarket type is consumed by normalizer.ts and utils.ts -- both are updated.
• mapMarketToUnified in utils.ts is called from mapQuestionToEvent (same file, line 90). No callers from index.ts or other modules.
• No SDK-layer changes needed -- this fix is at the raw data ingestion layer, before the pipeline reaches OpenAPI or SDK shims.

Findings

Test regression (blocking): The existing test fixture in core/test/normalizers/exchange-normalizers-2.test.ts:414 still sets questionId: 7 on the MyriadRawMarket fixture. Because the interface now has [key: string]: unknown, TypeScript won't flag this -- but the normalizer now reads raw.eventId, which is undefined in the fixture. The test at line 452 ("sets eventId from questionId") will fail:

Expected: '7'
Received: undefined

Fix: change the fixture from questionId: 7 to eventId: 7, and update the test description from "sets eventId from questionId" to "sets eventId from eventId" or similar.

PMXT Pipeline Check

• Venue normalizer: Updated (normalizer.ts + utils.ts)
• OpenAPI schema: Not affected (eventId is already in the UnifiedMarket schema)
• SDK shims: Not affected (no change to the unified output shape)

Semver Impact

patch -- bug fix only, no public API changes.

Risk

LOW with the test fix. The eventId field in the live Myriad API is currently always null per #556, so this change makes the code read the correct (albeit empty) field. No behavioral regression for end users.