[VL] Support format_number function

[yikf]

What changes are proposed in this pull request?

Velox already registers the sparksql format_number(number, decimalPlaces) function (thousands separators, fixed decimal digits, HALF_EVEN rounding), so wire it up on the Gluten side.

• https://facebookincubator.github.io/velox/functions/spark/string.html#format_number

Unlike CAST(number AS VARCHAR), this adds thousands separators and fixed decimal places. Supports tinyint, smallint, integer, bigint, float, and double.

How was this patch tested?

New UTs.

Was this patch authored or co-authored using generative AI tooling?

Yes, Assisted-by: Claude:claude-opus-4-8

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Pull request overview

This PR wires Spark SQL format_number(number, decimalPlaces) through Gluten’s Substrait expression mapping so Velox can execute it natively where supported, and adds coverage plus documentation/restriction metadata.

Changes:

• Add format_number to ExpressionNames and register FormatNumber in ExpressionMappings.
• Document Velox support as partial (numeric primitives only; DecimalType excluded) and add a corresponding restriction entry for doc generation.
• Add Velox backend validation tests for format_number, including expected Spark-side execution for DecimalType input.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
shims/common/src/main/scala/org/apache/gluten/expression/ExpressionNames.scala	Adds the FORMAT_NUMBER function name constant used across shims/mappings.
gluten-substrait/src/main/scala/org/apache/gluten/expression/ExpressionMappings.scala	Maps Spark’s FormatNumber expression to Substrait function name format_number.
docs/velox-backend-scalar-function-support.md	Updates function support table to reflect partial support + DecimalType restriction note.
backends-velox/src/test/scala/org/apache/gluten/functions/ScalarFunctionsValidateSuite.scala	Adds UTs covering offload for supported numeric types and non-offload for DecimalType input.
backends-velox/src/main/scala/org/apache/gluten/expression/ExpressionRestrictions.scala	Adds a FormatNumberRestrictions entry for doc/restriction reporting.
backends-clickhouse/src/main/scala/org/apache/gluten/utils/CHExpressionUtil.scala	Blacklists format_number for ClickHouse backend to avoid incorrect pushdown.

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Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.

Suppressed comments (1)

backends-velox/src/test/scala/org/apache/gluten/functions/ScalarFunctionsValidateSuite.scala:768

• The added test comments say this block exercises HALF_EVEN rounding and thousands separators, but the current inputs from TPCH columns are not deterministic for those edge cases (e.g., they may not hit tie-to-even cases, and values may not exceed 999 to guarantee separators). Adding deterministic cases (using datatab.double_field1 which already includes 1.025/1.035/1.045, and a scaled integer) will make the intent of the test reliably true.

    // Floating-point input, exercising HALF_EVEN rounding and thousands separators.
    runQueryAndCompare(
      "SELECT format_number(cast(l_quantity as double), 1) FROM lineitem limit 50") {
      checkGlutenPlan[ProjectExecTransformer]
    }

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