fix: [INFRA-673] Fix Jar properties extraction

[alexs-aero]

Brief

Currently, if a specific JAR parent-child artifact would be posted with empty fields, it could result in empty properties and render workflow corrupted.

This PR extracts Jar property/coordinate handling into separate helper and updates all calls to use extracted properties

Issue

INFRA-673
get_jar_metadata reads a sibling POM when a JAR has no embedded pom.properties. It previously read artifactId, version, and groupId as direct children of <project>, so child-module POMs that inherit coordinates from <parent> returned empty strings and overwrote values already parsed from the filename. Flat-layout child JARs could then structure and upload with blank or wrong GAV metadata.

Changes

• Extract shared Maven POM parsing into lib/maven-helpers.sh:
  • _maven_read_pom_coordinates — full GAV + packaging/module count, with <parent> fallback
  • _maven_read_pom_gav — GAV-only wrapper for callers that do not need packaging metadata
• Use pipe-delimited output so empty fields do not shift on read
• Update get_jar_metadata sibling-POM path to merge only non-empty POM values (filename-derived fields preserved when POM leaves them blank)
• Reuse the helper in type_detection.sh and entrypoint.sh pom-only upload path (same parent-inheritance bug)
• Remove duplicate XPath logic from type_detection.sh; simplify _pom_matches_flatten_resolved_heuristic to a single coordinate read
• Add make_flat_jar_with_pom test helper in maven_fixtures.bash
• Add docs/maven_lookup.md — resolution matrix, test coverage map, and known Stage 2 limitation for shaded uber JARs (first pom.properties in zip order; not fixed here)

Tests

Added to test_metadata.bats:

• get_jar_metadata reads name, version and group from a sibling POM — direct GAV on <project>
• get_jar_metadata keeps filename pkgname when sibling POM inherits all GAV from parent — parent-only POM, no child artifactId
• get_jar_metadata resolves child module GAV when sibling POM inherits groupId and version from parent — typical multi-module child shape
  Existing maven structuring / bundle-metadata bats tests continue to pass.