Fix AddAnnotationProcessor when parent matches by GAV alone

[MBoegers]

Summary

• Fix orphan modules being silently skipped when their external parent's GAV happens to also exist as a separately ingested LST artifact (MavenResolutionResult.parentPomIsProjectPom() is GAV-only).
• Verify each parent→child link by walking aggregator <modules> chains; treat the child as an orphan when no aggregator actually links them.
• Fill in missing <configuration> / <annotationProcessorPaths> when the matched plugin is pre-declared without it (previously AddPluginVisitor left such plugins untouched).

Problem

AddAnnotationProcessor produced zero changes on Maven modules that:

1. Have no in-reactor parent in their actual build, but coincidentally co-exist in the same LST as a POM that shares their parent's GAV (e.g. an externally-ingested corporate parent), or
2. Pre-declare maven-compiler-plugin without annotationProcessorPaths (e.g. only to set <source>/<target> or for inherited config).

In case 1, parentPomIsProjectPom() returns true purely because GAVs match, so the child is routed to the in-reactor-parent branch and never added to candidateOrphanPaths. In the visitor it is neither parent nor orphan and is skipped, so its own build/plugins is never updated.

In case 2, AddPluginVisitor's "plugin already exists" branch leaves the plugin untouched, and the path-adding inner visitor only operates on existing <annotationProcessorPaths> tags — so nothing is ever added.

Solution

1. Reactor-aware parent classification. During scan, record tentativeChildToParent links and aggregator <modules> membership (read off the requested Pom, since ResolvedPom.subprojects is unreliable in some LST fixtures and mrr.getModules() lists POMs claiming this one as <parent> rather than ones aggregated by it). Before the visitor runs, walk aggregator <modules> chains via BFS; uphold a tentative link only when the child is reactor-reachable. Otherwise reclassify as orphan and add the processor to its own build/plugins. Skip dangling packaging=pom POMs that are neither claimed parents nor aggregators.

2. Bypass for GAV-coincident orphans. Pass the source path as AddPluginVisitor's filePattern only for the GAV-coincident orphan case so its own parentPomIsProjectPom() short-circuit doesn't refuse to add the plugin. Genuine in-reactor parents and true single-module orphans hit the existing acceptance path with filePattern=null.

3. Fill in missing <configuration> structure. Before the path-adding visitor runs, ensure the matched plugin has <configuration><annotationProcessorPaths/></configuration>. The helper is conservative: only fills in whichever level is missing.

Test plan

• New orphanModuleSharedLstWithExternalParent reproducer fails on main and passes after the fix.
• New addConfigurationToExistingPluginWithNoConfig and addAnnotationProcessorPathsToExistingConfiguration reproducers fail on main and pass after the fix.
• All 30 existing AddAnnotationProcessorTest cases continue to pass.
• Full :rewrite-maven:test passes.