Migrate to ColumnVector mergeAndSetValidity API for substring

[rishic3]

Contributes to rapidsai/cudf#23249. This depends on propagation of rapidsai/cudf#23030.

Description

This prepares cudf-spark for rapidsai/cudf#23030, which adds the ColumnVector.mergeAndSetValidity API and deprecates the ColumnView API to enforce zero-copy semantics on no-op merges.

substringColumn currently declares its string input as a ColumnView even though every caller supplies a ColumnVector. So this just changes the parameter type to ColumnVector to go through the new API. There is no change in runtime behavior. The existing test_substring and test_substring_column integration tests cover this path.

Checklists

Documentation

• Updated for new or modified user-facing features or behaviors
• No user-facing change

Testing

• Added or modified tests to cover new code paths
• Covered by existing tests
  (Please provide the names of the existing tests in the PR description.)
• Not required

Performance

• Tests ran and results are added in the PR description
• Issue filed with a link in the PR description
• Not required

[rishic3]

build

[greptile-apps]

Greptile Summary

Updates the private GPU substring helper to accept a ColumnVector for its string input, selecting the new cuDF ColumnVector.mergeAndSetValidity API while leaving substring behavior and call structure unchanged.

Confidence Score: 5/5

The PR appears safe to merge with no actionable defects identified.

The changed private parameter matches every existing call site, and the change does not alter the substring inputs, validity-mask construction, result ownership, or resource lifecycle.

Important Files Changed

Filename	Overview
sql-plugin/src/main/scala/org/apache/spark/sql/rapids/stringFunctions.scala	Narrows the private helper’s string parameter to ColumnVector; all existing callers already supply that type, with no identified behavioral or lifecycle regression.

_{Reviews (1): Last reviewed commit: "Use ColumnVector mergeAndSetValidity for..." | Re-trigger Greptile}

[ttnghia]

Approve, but we have to wait for the cudf changes to get in tomorrow before we can build this.