[FLINK-40528][table-planner] Address additional feedback for partial-delete StreamExecCalc

[twalthr]

What is the purpose of the change

Addresses some additional feedback on #29106.

Brief change log

Minor stability improvements (for stable compiled plan and additional safety net in case assumptions are violated).

Verifying this change

Only minor hotfixes.

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): 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?

• Yes (please specify the tool below)

Generated-by: Sonnet 5

[flinkbot]

CI report:

• 75b596d Azure: SUCCESS

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