fix: preserve concat binary string bytes

[manuzhang]

Which issue does this PR close?

Relates to #4180.

Rationale for this change

spark.sql.function.concatBinaryAsString=true makes Spark insert BinaryType-to-StringType casts before concat. Spark preserves malformed bytes in those casts, while Comet's native Arrow string path normalizes them to UTF-8 replacement bytes. For example, Spark keeps FFFE41, but Comet produced EFBFBDEFBFBD41.

What changes are included in this PR?

• Route concat trees containing BinaryType-to-StringType casts through Spark's generated code inside the Comet pipeline.
• Add a two-value SQL config matrix covering result bytes, malformed UTF-8, empty values, and nulls.
• Add a Scala assertion for result types and direct proof that both config values use the JVM codegen dispatcher without operator fallback.
• Record the supported behavior in the Spark configuration contributor guide.

How are these changes tested?

• make core
• Focused CometSqlFileTestSuite concat_binary_as_string on Spark 3.4.3, 3.5.8, 4.0.2, and 4.1.2
• Focused CometCodegenSuite concatBinaryAsString on the same versions
• ./mvnw spotless:check -Pspark-4.1 -Pjdk17 -DskipTests -Dscalastyle.skip=true
• Dynamic documentation generation for all four Spark profiles