Java SDK: Pack each native Dag's source file into the bundle JAR

[jason810496]

• depends on: Java SDK: Serialize native Dags to DagSerialization v3 #71190
• related: Add ADR for Lang SDK Dag and mixed-language Task processing flow #71929, TS SDK: embed one source region per native Dag file #73723
• Diff for early review: 56e0795...jason/java-sdk/pack-dag-sources

Merge order:

1. Java SDK: Group a native Dag's tasks with task groups #74230 — Group a native Dag's tasks with task groups
2. Java SDK: Serialize native Dags to DagSerialization v3 #71190 — Serialize native Dags to DagSerialization v3
3. Java SDK: Pack each native Dag's source file into the bundle JAR #74096 — Pack each native Dag's source file into the bundle JAR (current one)

Why

Airflow can parse native Java Dags (#74036), and its Code view needs the Dag's source. A bundle JAR carried at most one hand-picked file, so a Dag declared in another class had no source of its own. This follows #73723 for TypeScript: pack each Dag's source file once, map every Dag to its file, and always keep the entrypoint as a fallback.

What changes

• The Gradle plugin's new packDagSources task packs the source of mainClass and of each class that declares a Dag in Java into the bundle JAR, and indexes them in sources.json. Nothing to configure.
• To learn which class declares each Dag, the task runs mainClass once at build time, under a timeout. If that run fails or does not finish, the build logs a warning and packs only the mainClass source.
• Only the JARs a bundle is assembled from, jar and shadowJar, carry the sources and the Airflow-Java-SDK-Sources attribute, so a sources or javadoc JAR is not mistaken for a bundle.
• A Dag built by a @Builder.Dag class maps to that class, not to the generated builder. Task handlers for a Python Dag are not listed.
• .java, .kt and .scala sources all work.

The bundle JAR now holds:

META-INF/MANIFEST.MF
  Main-Class: org.apache.airflow.example.ExampleBundleBuilder
  Airflow-Java-SDK-Sources: META-INF/airflow/sources.json
META-INF/airflow/sources.json
META-INF/airflow/sources/org/apache/airflow/example/ExampleBundleBuilder.java
META-INF/airflow/sources/org/apache/airflow/example/nativedag/AnnotationExample.java
META-INF/airflow/sources/org/apache/airflow/example/nativedag/InterfaceExample.java

{
  "entrypoint_path": "org/apache/airflow/example/ExampleBundleBuilder.java",
  "dag_source_paths": {
    "java_native_annotation_example": "org/apache/airflow/example/nativedag/AnnotationExample.java",
    "java_native_interface_example": "org/apache/airflow/example/nativedag/InterfaceExample.java"
  }
}

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Was generative AI tooling used to co-author this PR?

• Yes, with help of Claude Code Opus 5.5 following the guidelines

[jason810496]

Thanks for the review.