Breaking change for v27: default to exhausted DataLoader dispatching - #4534

Breaking change — targeted for GraphQL Java 27. Do not backport to 26.x or earlier.

Use ExhaustedDataLoaderDispatchStrategy instead of PerLevelDataLoaderDispatchStrategy when automatic dispatching is enabled and no strategy is configured. Exhausted dispatching runs queued loads when the tracked fetching/completion work and batch loading are exhausted, and supports chained loads without an opt-in.

Compatibility and migration

• Dispatch timing and batch composition can change: batches are no longer tied to query levels.

• Obtain loaders through DataFetchingEnvironment.getDataLoader(...). The default now returns the tracking wrapper; loading directly through the registry or a separately held loader bypasses tracking and can leave requests waiting indefinitely.

• To retain per-level dispatching for a request:

  GraphQL.unusualConfiguration(executionInput)
          .dataloaderConfig()
          .enableDataLoaderExhaustedDispatching(false);

• Existing enableDataLoaderChaining(true) continues to select per-level chained dispatching when exhausted dispatching is unset. Explicitly enabling both strategies still fails. Disabling automatic dispatch and requests without a supplied registry still use the no-op strategy.

Implementation and tests

Update strategy selection, loader wrapping, the configuration getter, and Javadoc together. Add selection coverage for queries, mutations, and subscriptions; verify the effective configuration on context builders and contexts, default chained loading, loader wrapping, and the explicit per-level migration path. Migrate the hanging-test fixture to tracked loader access and test it with both strategies. Update the deferred batching assertion to account for the new default's timing.

Validation: ./gradlew --no-daemon test testng jacocoTestReport javadoc passed on Java 25. No per-class coverage regressions against test-baseline.json using the CI comparison rules.