[SPARK-56811][UI] Restore sub-execution grouping on the SQL tab listing

[yaooqinn]

What changes were proposed in this pull request?

This PR restores sub-execution grouping on the SQL tab listing that was silently dropped when the listing was switched to server-side pagination in SPARK-56140.

• Backend (SqlResource.sqlTable): accept a new groupSubExecution query parameter (default = cluster config spark.ui.groupSQLSubExecutionEnabled). When enabled, paginate over root executions only and embed each root's children as subExecutions: [...] on the row. Sub-executions whose root is missing from the filtered set surface as roots so they are never hidden. Fix recordsTotal / recordsFiltered to count root rows in grouped mode so the DataTables "Showing X to Y of Z entries" matches the visible rows.
• Frontend (allexecutionspage.js): read the existing group-sub-exec-config data attribute, forward groupSubExecution=true|false to the server, and append a trailing Sub Executions column showing a +N sub toggle on roots that have children. Toggling expands the row using DataTables row().child() with a nested table — matches the SPARK-41752 / Spark 4.1 layout.
• CSS (webui-dataTables.css): minimal styling for the toggle link and the nested child table (indent + tertiary background).

Why are the changes needed?

SPARK-41752 introduced sub-execution grouping in Spark 4.1, and SPARK-55875 carried it over when the listing moved to client-side DataTables. SPARK-56140 then switched the listing to server-side pagination but didn't carry over the grouping logic — every sub-execution now shows up as its own flat row, regressing the UX for queries such as CACHE TABLE and nested CTAS.

Does this PR introduce any user-facing change?

Yes — the SQL tab listing again folds sub-executions under their root, with a +N sub toggle to expand. Default behaviour is controlled by the existing spark.ui.groupSQLSubExecutionEnabled config (default true).

The screenshots below were taken from the same workload (SELECT, CACHE TABLE, broadcast join, DROP TABLE, CREATE TABLE … AS) — CACHE TABLE and the inner CREATE TABLE each spawn a sub-execution.

Before — flat listing (today, post-SPARK-56140): sub-executions appear as flat sibling rows; e.g. id 2/3/4 are all CACHE TABLE people_eng_cached and id 7/8/9 are all nested CTAS.

After — grouped, collapsed: only roots are listed, with a trailing +1 sub toggle on rows that own a sub-execution.

After — grouped, expanded: clicking +1 sub expands a nested table inline.

How was this patch tested?

• New unit test in SqlResourceWithActualMetricsSuite covering both grouped and flat modes against a session that runs CACHE TABLE (which produces a root + sub-execution pair).
• Existing SqlResourceWithActualMetricsSuite and AllExecutionsPageWithInMemoryStoreSuite continue to pass.
• Manual verification in a local Spark UI (screenshots above): ran the same workload twice — once with spark.ui.groupSQLSubExecutionEnabled=false (flat) and once with the default true (grouped) — and confirmed both modes render correctly.

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