maven: decode versions from core=gav responses

[andrew]

The Maven client queries Maven Central using core=gav, whose result documents return the artifact version in v:

{"g":"org.apache.maven","a":"maven-core","v":"3.9.11","timestamp":1757293341000}

searchDoc.Version currently uses json:"latestVersion", so the version is empty after decoding a real response. This affects the version used by FetchPackage and FetchMaintainers, and causes FetchVersions to return entries with blank version numbers.

The existing tests serialize searchDoc, which produces the same incorrect latestVersion field and does not exercise the Maven Central response shape.

Please decode v for core=gav responses and add wire-format fixtures that verify non-empty versions through the public methods. This affects registries v0.6.4.