test(spark): characterize SparkToArrowSchema Spark-to-Arrow conversion

[jackylee-ch]

Rationale for this change

SparkToArrowSchema (java/vortex-spark/src/main/java/dev/vortex/spark/write/SparkToArrowSchema.java)
converts Spark SQL schemas to Arrow schemas on the write path, and currently has no
test coverage. It is the mirror image of ArrowUtils on the read path, which gained
characterization tests in #8770. Like that mapping table, this conversion is exactly the
kind of code that regresses quietly: a wrong bit width, a dropped timezone, or lost
nullability would silently corrupt written files rather than fail loudly.

What changes are included in this PR?

A new SparkToArrowSchemaTest that characterizes the conversion:

• Primitive mappings: Boolean → Bool; Byte/Short/Integer/Long → signed Int
  8/16/32/64; Float/Double → SINGLE/DOUBLE floating point; String → Utf8;
  Binary → Binary; Decimal → 128-bit Arrow Decimal preserving precision and
  scale; Date → Date(DAY); Timestamp → Timestamp(MICROSECOND, "UTC") and
  TimestampNTZ → Timestamp(MICROSECOND, null).
• Schema structure: field names and ordering are preserved; field-level nullability
  is carried over to the Arrow field.
• Nested types: StructType converts to a Struct field with recursively converted
  children (including per-child nullability); ArrayType converts to a List whose
  element child carries containsNull.
• Rejected types: unsupported Spark types (e.g. CalendarIntervalType) raise
  UnsupportedOperationException naming the offending type.

This is a test-only change; no production code is modified. 12 tests, all passing
locally (:vortex-spark_2.12:test --tests 'dev.vortex.spark.write.SparkToArrowSchemaTest').

What APIs are changed? Are there any user-facing changes?

None. No production code or public API is touched.