Normalize item map parsing in RewardGui and add GitHub Actions jar build

[iphonetips1]

Motivation

• Fix issues when reward item definitions contain nested maps or lists (including enchantment maps) so ItemBuilder.create receives a normalized map shape.
• Add a simple CI workflow to produce and upload the plugin jar on pushes and PRs.

Description

• Added normalizeItemMap to RewardGui which deeply converts nested maps and list elements into a shape ItemBuilder expects, including converting single-entry maps in lists to key:value strings and converting enchantment maps to a list of key:value strings, and uses LinkedHashMap to preserve ordering.
• Replaced the previous raw cast in the claim handling code with ItemBuilder.create(normalizeItemMap(map)).get() to avoid ClassCastException and support richer configuration forms.
• Added a GitHub Actions workflow .github/workflows/build-jar.yml that sets up Java 21 with Maven caching, runs mvn -B -DskipTests package, and uploads target/*.jar as an artifact.

Testing

• CI workflow Build AxRewards Jar is provided to run mvn -B -DskipTests package on ubuntu-latest and upload the produced jar; tests are explicitly skipped via -DskipTests so no unit tests were executed in CI.
• No automated unit tests were run as part of this change.

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Codex Task