chore: publish java fixture to GitHub Packages

[jimisola]

Context

Part of the native-package axis of the parameterized regression test matrix. Publish the Java fixture Maven artifact to the GitHub Packages registry of reqstool/reqstool-regression so reqstool-client's MavenLocation can consume it in integration tests.

Proposed approach

• Configure fixtures/ecosystems/java/pom.xml to publish a -reqstool classifier artifact (matches MavenLocation's default classifier)
• Add a publish workflow to .github/workflows/ that calls the reusable java-publish-to-maven.yml with working-directory: fixtures/ecosystems/java
• Publish target: GitHub Packages of reqstool/reqstool-regression (never Maven Central — these are test fixtures only)

Acceptance criteria

• fixtures/ecosystems/java/pom.xml configured to publish a reqstool classifier to GH Packages of reqstool/reqstool-regression
• Publish workflow wired up and triggering on releases or manual dispatch
• Artifact consumable via reqstool-client MavenLocation pointing at the GH Packages registry

Dependencies

• Depends on: feat(workflows): add working-directory input to reusable build/publish workflows .github#35 (reusable workflow needs working-directory input)

Moved from reqstool-client#388.