Support Maven 4 versionless parent references (parent inference)

[MBoegers]

Problem

Maven 4 (model version 4.1.0) allows omitting <version> from <parent> elements. The parent coordinates are inferred from the POM found at <relativePath> (defaulting to ..).

<parent>
  <groupId>com.example</groupId>
  <artifactId>parent</artifactId>
  <!-- version omitted — inferred from ../pom.xml -->
</parent>

Currently, ResolvedPom.resolveParentPom() throws MavenParsingException("Parent version must always specify a version") when the version is null. Since OpenRewrite only parses POMs from successful builds, a null parent version implies a valid Maven 4 project.

Current behavior

• Pom.getParent().getGav().getVersion() returns null
• resolveParentPom() throws immediately
• MavenPomDownloader.download() also rejects null version at line 485

Desired behavior

• If parent version is null, attempt reactor lookup via relative path + groupId/artifactId match
• If found, use the local parent POM (version inferred from it)
• If not found in reactor, throw MavenParsingException (no remote fallback without a version)

Scope

• ResolvedPom.resolveParentPom() — handle null version before the existing placeholder check
• MavenPomDownloader.download() — needs a reactor-only lookup path that accepts null version

Related

• Maven 4 parent inference: https://maven.apache.org/whatsnewinmaven4.html#Parent_inference
• MNG-624
• Fix Maven parser on ${project.version} in parent reference #6860 laid the groundwork (reactor-first lookup, relaxed path-based matching)