[FLINK-40288][table] Use correct type in ExprCodeGenerator#visitFieldAccess

[snuyanzin]

What is the purpose of the change

The PR fixes JoinSemanticTests behavior for nested types

Brief change log

ExprCodeGenerator

Verifying this change

JoinSemanticTests

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: [Tool Name and Version]

[flinkbot]

CI report:

• d7428fc Azure: SUCCESS

Bot commands

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[twalthr]

LGTM