[FLINK-39226][python] Fix embedded PyIterator class cast after recovery

[bowenli86]

What is the purpose of the change

This pull request fixes FLINK-39226 by avoiding direct casts to pemja.core.object.PyIterator in embedded Python operators. After recovery, the returned iterator object can be loaded by a different user-code classloader, so hard casts can fail with ClassCastException. The change introduces a reflective wrapper that can consume iterator-like results without depending on the local PemJa class identity.

Brief change log

• Add EmbeddedPythonIterator as a reflective adapter for embedded Python result iterators
• Replace direct PyIterator casts in embedded one-input, two-input, keyed timer, window, and table operators
• Add tests that reproduce the cross-classloader cast failure and verify the new iterator wrapper

Verifying this change

Please make sure both new and modified tests in this PR follow the conventions for tests defined in our code quality guide.

This change added tests and can be verified as follows:

• Added EmbeddedPythonIteratorTest to cover foreign-classloader iterators and direct-cast failure reproduction
• Ran ./mvnw -pl flink-python -am -Dfast -DskipITs -Dtest=EmbeddedPythonIteratorTest -Dsurefire.failIfNoSpecifiedTests=false test

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): yes
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: yes
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• f96b414 Azure: SUCCESS

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[featzhang]

Added iterator implementation for testing, enabling cross-classloader behavior validation.

[bowenli86]

@featzhang thanks for reviewing
@dianfu can you help review as well?