maven: decode the version from the Solr v field on core=gav responses

[frankreyesgarcia]

Fixes #49.

FetchVersions and FetchPackage query Maven Central with core=gav, where each result document carries the concrete version in v:

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

searchDoc.Version was tagged json:"latestVersion" — a field that only exists on the default (non-gav) core, where a document aggregates an artifact. Decoding a real core=gav response therefore left Version empty, so FetchVersions returned entries with blank Number, and FetchPackage/FetchMaintainers resolved the effective POM against an empty version.

The existing tests encode a searchDoc struct into the mock response and decode it with the same struct, so the wrong field name round-tripped and was never exercised.

Change

• searchDoc.Version → json:"v".
• New TestFetchVersionsParsesRealSolrGavShape feeds the raw JSON the search API actually returns and asserts non-empty versions through FetchVersions.

Before / after (against real Maven Central)

before:  FetchVersions("com.google.guava:guava")  -> n=150, every Number == ""
after:   FetchVersions("com.google.guava:guava")  -> n=150, Number populated, contains "33.0.0-jre"
         FetchVersions("org.slf4j:slf4j-api")     -> n=106, Number populated, contains "2.0.13"

Full go test ./... passes.

🤖 Generated with Claude Code

[algomaster99]

@andrew e4cbda5 (this PR) did you automate this commit or did you trim it manually? I like this if it is automated.