[runtime][python] Release Pemja objects when closing action executor

[joeyutong]

Linked issue: #942

Purpose of change

PythonActionExecutor owns the Pemja PyObject handles for its Python async thread pool and runner context. The existing close path invokes Python-level cleanup but does not call PyObject.close(), which performs Pemja's native decRef. Repeated task restarts can therefore retain old runner contexts and their Flink task graphs through JNI global references.

This change:

• runs Python-level cleanup before releasing each owned PyObject;
• releases both handles even if one cleanup operation fails;
• preserves the first failure and attaches later failures as suppressed exceptions; and
• clears the fields so repeated close calls are idempotent.

Tests

• mvn -B --no-transfer-progress -pl runtime -am -Dtest=PythonActionExecutorTest -Dsurefire.failIfNoSpecifiedTests=false test (2 passed)
• Maven Spotless formatting/checks passed as part of the Java verification
• Local 20-restart failover A/B: post-Full-GC heap growth changed from 29.2 MiB to 1.7 MiB, and old runner-context/task graphs were no longer retained after termination

API

No public API changes.

Documentation

• doc-needed
• doc-not-needed
• doc-included