Spark 4.2: segment-tree window metrics unavailable under CometWindowExec #4964

Spark 4.2 adds a segment-tree window-aggregation optimization to WindowExec, along with metrics such as "number of segment-tree frames prepared" and "number of segment-tree fallback frames prepared", plus an eligibility allowlist.

Comet replaces WindowExec with CometWindowExec, which has no segment-tree implementation and does not expose those metrics. The new test suites read the metric from a Spark Window node in the executed plan and fail with "No Window node in plan" when Comet is enabled.

Affected Spark SQL test suites (currently marked IgnoreCometSuite in dev/diffs/4.2.0.diff):

• SegmentTreeWindowMetricsSuite
• WindowSegmentTreeAllowlistSuite