[FLINK-40672][table] Preserve nullability when casting VARIANT values

[dylanhz]

What is the purpose of the change

Fix FLINK-40672: CAST(PARSE_JSON('null') AS INT) should return SQL NULL. A non-null VARIANT can contain JSON null, but deriving CAST nullability solely from the source type can produce a non-null result type and cause the cast to fail.

Brief change log

• Apply Calcite's VARIANT nullability rules to Flink's SQL CAST type derivation.
• Make Table API casts from VARIANT to other types nullable and use the inferred result type in planner conversion. Preserve VARIANT-to-VARIANT and nested target nullability.
• Extend the existing type strategy and CAST integration tests.

Verifying this change

• TypeStrategiesTest: 34 tests passed.
• CastFunctionITCase: 951 tests passed using JUnit and MiniCluster, including SQL, Table API, and Table API-to-SQL serialization.
• Spotless and Checkstyle passed for flink-table-common and flink-table-planner.

The full repository clean verify suite was not run.

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

• Dependencies: no.
• The public API: yes; corrects the inferred nullability of CAST results. No API signatures change.
• The serializers: no.
• The runtime per-record code paths: yes; corrected nullability enables the existing VARIANT-null handling in generated CAST code.
• Deployment or recovery: 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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Generated-by: Codex (GPT-6)

[flinkbot]

CI report:

• a28526f Azure: SUCCESS

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[twalthr]

Thank you for catching this @dylanhz. @raminqaf is out of office this week, but will take a look after that.