Bind mapped stub-task arguments in the Java SDK runtime

### Background https://github.com/apache/airflow/pull/70570 makes the execution API derive a **per-map-index** arg-binding spec for a dynamically mapped `@task.stub`, so a foreign runtime receives *its* element rather than the aggregated upstream output. https://github.com/apache/airflow/pull/70571 implements the Go-SDK consumer; the Java SDK has no equivalent. `XComArgBinding` carries two selectors the unmapped path never sets: - `map_index` — pull the upstream XCom row at that index (expand over a *mapped* upstream). - `element_index` — pull the unmapped upstream row, then take element N of the resulting sequence (expand over an *unmapped* upstream's list output). Literal expands are resolved to their element server-side and arrive as plain literals, so the binding surface itself is unchanged from the unmapped case. ### What needs to happen 1. Extend the Java SDK's arg-binding resolution to honour `map_index` and `element_index`, with typed and out-of-range errors that fail the task before its body runs. 2. Keep the unmapped path unchanged: a binding with neither selector takes the whole value. 3. Add an example mapped Dag to the Java example bundle and cover it with an E2E test. ### Acceptance criteria - A `@task.stub` expanded with `.expand()` delivers the correct per-index element to the Java task for all three expand forms (literal collection, unmapped upstream's list output, mapped upstream). - Multiple expanded kwargs (the cross product) bind correctly, including mixed expand sources in one call. - `.partial()` literals and unmapped-upstream XComs bind identically on every index. - Out-of-range or type-mismatched selectors fail the task loudly rather than silently zero-filling. ### Context - Depends on https://github.com/apache/airflow/pull/69757 and https://github.com/apache/airflow/pull/70570 - Go parallel: https://github.com/apache/airflow/pull/70571 - Java-side arg binding for unmapped stub calls: https://github.com/apache/airflow/pull/71057