feat: support TimestampType join keys in SortMergeJoin

[andygrove]

Which issue does this PR close?

Closes #3985.

Rationale for this change

CometSortMergeJoinExec rejected TimestampType join keys via supportedSortMergeJoinEqualType, forcing a fall-back to Spark whenever a SortMergeJoin keyed on a timestamp column. The restriction was historical — TimestampNTZType was already allowed but TimestampType had never been enabled.

Naively flipping the guard surfaces an upstream limitation: DataFusion's SMJ comparator (joins/sort_merge_join/stream.rs is_join_arrays_equal, joins/utils.rs compare_join_arrays) only pattern-matches Timestamp(_, None). Spark's TimestampType serializes to Timestamp(µs, "UTC"), hitting the fallthrough not_impl_err!. Because Spark stores TimestampType as UTC-normalized microseconds, casting Timestamp(µs, Some(_)) → Timestamp(µs, None) is a metadata-only operation that preserves both equality and row order, so it is safe to apply at the join-key level.

What changes are included in this PR?

• JVM guard: add TimestampType to supportedSortMergeJoinEqualType in operators.scala.
• Native planner workaround: in the OpStruct::SortMergeJoin arm of planner.rs, introduce a strip_timestamp_tz helper that wraps any Timestamp(_, Some(_)) join-key expression in a CastExpr to Timestamp(_, None) before handing the keys to SortMergeJoinExec::try_new. sort_options and the child plans are untouched.
• Tests: replace the obsolete "falls back to Spark" test in CometJoinSuite with four positive tests exercising the new path:
  • Inner join with Asia/Kathmandu session timezone.
  • LEFT OUTER, RIGHT OUTER, FULL OUTER variants.
  • Composite (string, timestamp) join key.
  • Nullable timestamp keys (both inner and full-outer variants).

How are these changes tested?

All four new tests use checkSparkAnswerAndOperator with classOf[CometSortMergeJoinExec] to assert both correctness against Spark and native execution. The full CometJoinSuite passes (13/13). cargo clippy --all-targets --workspace -- -D warnings is clean and make format reports no changes.