Fix OSGi Export-Package for shaded Guava packages

[kevinherron]

What problem(s) was I solving?

Fixes #1688 - OSGi bundle deployment fails with constraint violations due to mismatched package imports/exports for shaded Guava packages.

When deploying Milo as an OSGi bundle, sdk-client declares an import for org.eclipse.milo.shaded.com.google.common.collect, but stack-core only exports com.google.common.collect (the unshaded name). This happens because maven-bundle-plugin generates the MANIFEST.MF in the process-classes phase, before maven-shade-plugin relocates the Guava packages in the package phase.

What user-facing changes did I ship?

OSGi deployments of Milo should now work correctly without constraint violations for the shaded Guava packages.

How I implemented it

Added explicit maven-bundle-plugin configuration in stack-core/pom.xml using the _exportcontents BND instruction. This instruction adds packages to the OSGi Export-Package manifest header without requiring the packages to exist at analysis time, allowing stack-core to declare exports for the shaded packages that maven-shade-plugin creates later in the build lifecycle.

How to verify it

Manual Testing

1. Build the project: mvn clean package
2. Inspect stack-core's MANIFEST.MF to verify it now includes Export-Package entries for org.eclipse.milo.shaded.com.google.common.*
3. Deploy the bundles in an OSGi container and verify no constraint violations occur

Description for the changelog

Fixed OSGi Export-Package manifest to include shaded Guava packages, resolving bundle deployment constraint violations.