fix: fix sunset ls to report all sunsetting versions of a resource

[andmatei]

Summary

• contentExtensions previously only returned extensions from the oldest API version, assuming older versions always have the earliest sunset date
• This doesn't hold when newer versions have earlier sunsets (e.g. due to delayed older versions ensuring v1/v2 parity)
• Now all content entries with an x-sunset extension are returned, so sunset ls lists every version with a sunset independently

Test plan

• Updated unit tests for contentExtensions / successResponseExtensions to return []map[string]any
• Added "Multiple versions with sunset extensions" test case to TestNewSunsetListFromSpec
• Added "Content entry without sunset extension is skipped" test case
• Existing TestList_Run integration test passes unchanged