What changes were proposed in this pull request?

This PR addresses Spark 4.0 compatibility issues and refactors the ExpressionConverter to improve code quality.

Main Changes:

1. Add compatibility layer for StructsToJson expression across Spark versions

   • In Spark 4.0, StructsToJson was replaced with Invoke expression using StructsToJsonEvaluator
   • Added StructsToJsonInvoke extractor in shims for all Spark versions (3.2-4.0) to provide a unified interface
   • For Spark 4.0: Extracts evaluator options, child expression, and timeZoneId from the Invoke pattern
   • For Spark 3.2-3.5: Returns None to maintain backward compatibility

2. Add support for StaticInvoke expressions in Spark 4.0

   • lengthOfJsonArray and jsonObjectKeys now use StaticInvoke in Spark 4.0
   • Added mappings to handle these functions properly in ExpressionConverter

3. Refactor ExpressionConverter to reduce return usage

   • Restructured code to avoid explicit return statements, following Scala best practices
   • Improved readability by using pattern matching and Option-based control flow
   • Extracted common mappings (icebergStaticInvokeMap, staticInvokeMap) to reduce code duplication

Why are the changes needed?

• Spark 4.0 introduced breaking changes where StructsToJson was replaced with Invoke expressions
• Without this compatibility layer, JSON-related functions fail in Spark 4.0
• The return keyword is discouraged in Scala as it breaks functional programming style and can lead to unexpected behavior

Does this PR introduce any user-facing change?

No. This is an internal compatibility fix.

How was this patch tested?

• Removed Spark-version-specific test limitations in JsonFunctionsValidateSuite
• Existing JSON function tests now pass across all Spark versions (3.2-4.0)

Related issue: #11088