[EXPERIMENTAL] feat(dag): migrate DAG support to extension SPI

[zhongkechen]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Warning

Experimental stacked PR. This work depends on draft PR #607 and is not intended for production use. The DAG API, checkpoint shape, and implementation may change before merge or release.

Issue Link, if available

Stacked on #607.

Description

Migrates the experimental DAG implementation to the extension operation SPI introduced by #607.

• Adds static DurableDagOperation.dag(...) and dagAsync(...) entry points following the new Durable*Operation facade style.
• Removes DAG-specific methods and internal hooks from DurableContext.
• Schedules DAG containers and tasks through ExtensionContext and ExtensionOperation.
• Preserves stable name-based DAG task IDs.
• Reuses unchanged map, parallel, and wait-for-condition facades through a DAG-internal reserved-context adapter.
• Leaves all other Durable*Operation classes and their implementation tests unchanged from the stacked base.
• Handles large DAG results with extension child replay state.
• Updates examples, conformance handlers, integration tests, and DAG documentation.

Demo/Screenshots

Not applicable. This is an SDK API and execution implementation change.

Checklist

• I have filled out every section of the PR template
• I have thoroughly tested this change

Testing

Unit Tests

Yes. Added focused coverage for DurableDagOperation and the DAG-internal reserved-context adapter.

Integration Tests

Yes. DAG integration and conformance tests were updated for the static operation facade. The complete Maven reactor passes.

Commands run:

mvn spotless:apply
mvn -pl sdk -Dtest=DurableDagOperationTest,ReservedOperationContextTest test \
  -DargLine=-javaagent:$HOME/.m2/repository/org/mockito/mockito-core/5.23.0/mockito-core-5.23.0.jar
mvn test \
  -DargLine=-javaagent:$HOME/.m2/repository/org/mockito/mockito-core/5.23.0/mockito-core-5.23.0.jar

Cloud example tests remain disabled by default.

Examples

Existing DAG examples were migrated to DurableDagOperation; no additional example was required.