`JavaCoordinator`: Rename `bundles_folder`, accept `list[str]`, and switch coordinator config to dict

[jason810496]

Summary

Several improvements to the JavaCoordinator configuration structure:

1. Rename bundles_folder — The current key name can be confused with Airflow's Dag bundle concept. It should be renamed to something more precise (e.g. jars_root) that reflects its actual purpose: pointing to directories containing Java JARs.

2. Accept list[str] instead of str — The config value should accept a list of directory paths (or individual JAR files) rather than a single string, giving users more flexibility when managing classpaths across multiple locations.

3. Switch coordinator config from list to dict — The top-level coordinators config is currently a list of objects with a name key. Converting it to a dict keyed by coordinator name is more ergonomic and enables lazy initialization — coordinators defined in config won't incur any startup cost unless they are actually referenced by a task.

Proposed config shape

[sdk]
coordinators = {
    "jdk-17" = {
        "class" = "airflow.sdk.coordinators.java.JavaCoordinator",
        "kwargs" = {
            "java_executable" = "/usr/lib/jvm/java-17-openjdk/bin/java",
            "jars_root" = ["/files/lib", "/files/extra-lib"],
            "jvm_args" = ["-Xmx1024m"]
        }
    }
}

Motivation

• Avoids naming confusion with Dag bundles
• Supports multi-directory classpaths naturally
• Dict-keyed config is internally consistent with how coordinators are already looked up at runtime
• Enables lazy coordinator init as a free benefit

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Drafted-by: Claude Code (Opus 4.7); reviewed by @jason810496 before posting