[FLINK-39547] Fix table-planner class loading order

[piotrp]

What is the purpose of the change

Fix table planner class loading order so that deserialization of AdaptiveJoinGenerator uses classes table-planner.jar first, falling back to user classes only for missing classes.

Verifying this change

This change added tests and can be verified as follows:

• run JoinWithCustomTypeExampleTest test from the second commit - it will fail
• run all tests using HEAD of this PR - all tests will pass

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

• Dependencies: yes, a dependency in a test only subproject
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: yes, limited to AdaptiveJoinGenerator
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• 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

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

• No

[flinkbot]

CI report:

• 368459d Azure: SUCCESS

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[piotrp]

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