[Iceberg] Support nested column paths in keep/drop configuration

[PDGGK]

Summary

This PR enables support for nested column paths (dot notation) in both keep and drop configuration for Iceberg IO column pruning.

Problem:
Users cannot specify nested column paths like keep: ["data.name"] or drop: ["data.name"] - the validation fails with:

Invalid source configuration: 'keep' specifies unknown field(s): [data.name]

Solution:

1. Validation fix: Replace top-level-only column validation with Schema.findField() which natively resolves dot-notation paths
2. Drop fix: Use TypeUtil.indexByName() to enumerate all field paths (including nested), then select only leaf fields to prevent parent structs from implicitly including dropped children

Changes:

• IcebergScanConfig.java:
  • Updated validate() method to use findField() for nested path support
  • Rewrote resolveSchema() drop logic to handle nested paths correctly
• IcebergIOReadTest.java: Added test cases for nested column keep and drop

Fixes #37486

Test plan

• Added testNestedColumnPruningValidation - verifies nested paths in keep are accepted
• Added testNestedColumnDropValidation - verifies nested paths in drop work correctly
• Existing tests should continue to pass

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🤖 Generated with Claude Code

[gemini-code-assist]

Summary of Changes

Hello @PDGGK, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the Iceberg IO functionality by enabling users to specify nested column paths in the keep and drop configurations. This change improves the flexibility of column pruning and allows users to select specific nested fields within their Iceberg tables. The update includes a modification to the validation logic and the addition of a new test case to ensure the correct handling of nested column paths.

Highlights

• Nested Column Support: This PR introduces support for specifying nested column paths (using dot notation) in the keep and drop configurations for Iceberg IO column pruning.
• Validation Update: The validation logic in IcebergScanConfig.java has been updated to use Schema.findField() to resolve nested column paths, allowing for more flexible column selection.
• New Test Case: A new test case, testNestedColumnPruningValidation, has been added to IcebergIOReadTest.java to verify that nested column paths are correctly handled.

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Changelog

• IcebergIOReadTest.java
  • Added a new test case, testNestedColumnPruningValidation, to verify nested column path validation.
• IcebergScanConfig.java
  • Updated the validate() method to use findField() for resolving nested column paths.

Activity

• The PR introduces support for nested column paths in Iceberg IO column pruning.
• The validation logic in IcebergScanConfig.java is updated to use Schema.findField().
• A new test case is added to verify the functionality.

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[github-actions]

Checks are failing. Will not request review until checks are succeeding. If you'd like to override that behavior, comment assign set of reviewers

[PDGGK]

The failing beam_PreCommit_Java check appears to be unrelated to this PR's changes - the failures are in FileIO, CountingSource, TFRecordIO tests while this PR only modifies IcebergScanConfig.java. Could a maintainer please re-run the failed check? /retest