Java SDK: Clearer error when a task class cannot be instantiated

[FrankYang0529]

Why

• A Task class without a public no-argument constructor or a non-static inner class fails with a generic Error executing task plus a raw reflection trace, which steering Dag authors toward debugging execute() logic that never ran.

How

• Split instantiation from execution in TaskRunner.runTask: instantiation failures now log Cannot instantiate task class. A task class must be concrete and declare a public no-argument constructor with a taskClass field.
• Removed the redundant e.printStackTrace(). The structured log record already carries the full trace.
• Documented the constructor requirement in the interface-API section of java.rst.

Verification

• ./gradlew :sdk:test --tests "org.apache.airflow.sdk.execution.TaskTest"
• E2E_TEST_MODE=java_sdk uv run --project airflow-e2e-tests pytest "tests/airflow_e2e_tests/java_sdk_tests/test_java_sdk_dag.py::TestJavaSDKUninstantiableExample" -xvs

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

• Yes - Claude Code

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[jason810496]

Nice! LGTM overall.