[VL] Remove redundant string trimming before Velox casts

[rui-mo]

What changes are proposed in this pull request?

This patch removes the extra string trimming in Gluten before delegating casts to Velox.

Velox already applies Spark-compatible string cast trimming in its Spark cast hooks.

The corresponding Velox behavior is tracked with SPARK-59182, which discusses whether Spark should align the behavior of the two string cast trimming paths.

How was this patch tested?

Unit tests

[github-actions]

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[Copilot]

🟡 Changes recommended

The build helper script now defaults to applying a specific upstream Velox PR patch, which can unintentionally affect reproducibility and CI stability.

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Pull request overview

This PR removes Gluten-side string trimming logic that was previously injected before CAST-from-varchar when running on the Velox backend, relying instead on Velox’s Spark-compatible cast trimming behavior.

Changes:

• Removes the Velox-specific CAST trim-node injection path (and its plumbing) from Gluten’s cast conversion/execution APIs.
• Deletes the associated Velox config (spark.gluten.velox.castFromVarcharAddTrimNode) and removes it from documentation.
• Updates Spark 3.5 / 4.0 / 4.1 Gluten UTs to validate whitespace-tolerant casts without enabling the removed config.

File summaries

File	Description
gluten-ut/spark41/src/test/scala/org/apache/spark/sql/GlutenDataFrameSuite.scala	Drops use of the removed Velox trim config while keeping cast-with-whitespace assertions.
gluten-ut/spark40/src/test/scala/org/apache/spark/sql/GlutenDataFrameSuite.scala	Same test update for Spark 4.0 profile.
gluten-ut/spark35/src/test/scala/org/apache/spark/sql/GlutenDataFrameSuite.scala	Same test update for Spark 3.5 profile.
gluten-substrait/src/main/scala/org/apache/gluten/expression/ExpressionConverter.scala	Stops generating a backend-modified Cast for trimming; uses the original Cast child directly.
gluten-substrait/src/main/scala/org/apache/gluten/backendsapi/SparkPlanExecApi.scala	Removes the now-unused genCastWithNewChild hook from the backend API.
ep/build-velox/src/get-velox.sh	Sets a non-empty default UPSTREAM_VELOX_PR_ID, which forces patch application in builds.
docs/velox-configuration.md	Removes documentation for the deleted Velox trimming config.
backends-velox/src/main/scala/org/apache/gluten/config/VeloxConfig.scala	Removes the config accessor and registry entry for CAST-from-varchar trim-node injection.
backends-velox/src/main/scala/org/apache/gluten/backendsapi/velox/VeloxSparkPlanExecApi.scala	Deletes Velox’s override that injected StringTrim nodes ahead of casts.

Review details

• Files reviewed: 9/9 changed files
• Comments generated: 1
• Review effort level: Lite

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