[GR-79039] Restore aggregate resource filesystem lookup.

[graalvmbot]

Summary

• Treat bare resource:/ filesystems as aggregate views instead of binding them to concrete root 0.
• Preserve strict isolation for root-qualified resource URIs.
• Convert aggregate paths to URIs using the concrete root that supplied the selected first resource variant.
• Add regression coverage for a resource contributed by a different classpath root.

Motivation

Preserving resource roots caused an unqualified resource filesystem to inherit concrete root 0. As a result, a lookup failed when the requested resource was contributed by another classpath root. Bare resource:/ filesystems historically provide aggregate, first-match behavior, while explicitly root-qualified resource URIs must remain isolated.

Fixes #13920

Testing

• mx build
• mx checkstyle
• mx native-unittest com.oracle.svm.test.NativeImageResourceFileSystemProviderTest
• mx native-unittest com.oracle.svm.test.NativeImageResourceTest
• Built the Quarkus Liquibase native integration reproducer with the local Native Image and verified that both affected endpoints return HTTP 200.