feat: support StringSplitSQL for split_part

[michaelmitchell-bit]

Which issue does this PR close?

Closes #4561.

Rationale for this change

split_part lowers to element_at(StringSplitSQL(str, delimiter), partNum). Comet already supports element_at, but the inner StringSplitSQL expression was unsupported, so split_part fell back to Spark.

What changes are included in this PR?

Adds native support for Spark 4.x StringSplitSQL via a literal-delimiter split_sql function. Unlike split / StringSplit, this splits on the delimiter literally rather than as a regex and keeps trailing empty parts.

This also fixes ListExtract default-value typing so defaults are cast to the array element type rather than the list type. That is needed for split_part, which uses an empty-string default through element_at.

Updates expression support docs and adds SQL-file coverage for split_part, including literal delimiters, dynamic delimiters, nulls, out-of-range defaults, negative positions, and invalid zero index behavior.

How are these changes tested?

Ran:

• cargo fmt -p datafusion-comet-spark-expr
• cargo test -p datafusion-comet-spark-expr string_funcs::split
• cargo test -p datafusion-comet-spark-expr list_extract
• cargo build
• ./mvnw test -Dtest=none -Dsuites="org.apache.comet.CometSqlFileTestSuite split_part" -Dscalastyle.skip=true
• ./mvnw compile -Pspark-4.0 -DskipTests -Dscalastyle.skip=true
• ./mvnw compile -Pspark-4.2 -DskipTests -Dscalastyle.skip=true
• git diff --check

[andygrove]

Thanks @michaelmitchell-bit. This looks good overall.

StringSplitSQL is collation-aware in Spark and Comet does not support different collations yet, so it would be good to add a fallback for that case. See concat and reverse for examples.

[michaelmitchell-bit]

Thanks @michaelmitchell-bit. This looks good overall.

StringSplitSQL is collation-aware in Spark and Comet does not support different collations yet, so it would be good to add a fallback for that case. See concat and reverse for examples.

awesome, ty. added a fallback for non-utf8_binary collations and covered it in the split_part SQL file test.