fix: use camelCase endDateIso in polymarket normalizer

[realfishsam]

Fixes #557

[realfishsam]

PR Review: VERIFIED

What This Does

Fixes the Polymarket normalizer fallback for resolution date: changes market.end_date_iso (snake_case) to market.endDateIso (camelCase) to match the actual Gamma API response field name confirmed in the vendor OpenAPI spec (core/specs/polymarket/PolymarketGammaAPI.yaml line 1532).

Blast Radius

• Polymarket exchange: utils.ts (mapMarketToUnified) -- single line change in fallback path
• Only affects markets where market.endDate is falsy and endDateIso is the only available date field
• No SDK, OpenAPI, or type changes

Findings

1. Vendor spec confirms camelCase -- PolymarketGammaAPI.yaml defines endDateIso (camelCase), validating this fix.
2. Raw type interface is stale -- PolymarketRawMarket in fetcher.ts (line 55) still defines end_date_iso?: string (snake_case). This interface should be updated to endDateIso to stay consistent with the vendor spec and this fix. The field is accessed via [key: string]: unknown index signature so it works at runtime, but the typed property is now dead code.
3. No test for this fallback path -- the endDateIso fallback is only hit when endDate is missing, which appears to be an edge case. No test was added or exists.

PMXT Pipeline Check

• Field propagation: N/A (resolutionDate is already in all layers)
• OpenAPI sync: N/A
• Type safety: ISSUE -- PolymarketRawMarket.end_date_iso in fetcher.ts should be renamed to endDateIso to match

Semver Impact

patch -- bug fix for incorrect field name in fallback path

Risk

The stale end_date_iso property in PolymarketRawMarket interface is cosmetic but could mislead future contributors. Low risk since the fix is correct per the vendor spec.