Rationale for this change

PartitionPathUtils (java/vortex-spark/src/main/java/dev/vortex/spark/read/PartitionPathUtils.java)
discovers Hive-style partition columns from file paths and materializes them as constant
column vectors on the read path. It currently has no test coverage, despite encoding
several subtle behaviors: URL decoding of keys and values, the
__HIVE_DEFAULT_PARTITION__ null sentinel, first-= splitting, type inference
precedence (int → long → double → boolean → string), and per-type parsing into
ConstantColumnVector. This continues the characterization-test series from #8770
(read-path ArrowUtils) and #8782 (write-path SparkToArrowSchema).

What changes are included in this PR?

A new PartitionPathUtilsTest covering all three public methods:

• parsePartitionValues: extracts ordered key=value segments from paths; ignores
  segments without the key=value shape (including empty keys or values); URL-decodes
  both keys and values (New%20York → New York); splits on the first = so values
  may themselves contain =.
• inferPartitionColumnType: infers Integer for int-width values, Long for
  wider integrals, Double for decimal-point values, Boolean for case-insensitive
  true/false, and falls back to String (including for null and the
  __HIVE_DEFAULT_PARTITION__ sentinel).
• createConstantVector: null and the Hive default-partition sentinel produce null
  vectors; string/int/long/short/byte/boolean/float/double values are parsed into their
  target types; DateType parses as days-since-epoch (int) and both timestamp types as
  micros-since-epoch (long); unrecognized target types fall back to UTF8 strings.

This is a test-only change; no production code is modified. 19 tests, all passing
locally (:vortex-spark_2.12:test --tests 'dev.vortex.spark.read.PartitionPathUtilsTest').

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

None. No production code or public API is touched.