Lite's Job History shows that it is loading and says when it shows only the newest rows (#4478)

[erikdarlingdata]

Refs #4478.

Why

The Viewer's Job History tab (#4481) got a loading message while its read is in flight and a "showing the newest 2,000" label when the unfiltered read hit its row cap, so an empty grid mid-load doesn't read as "no jobs ran," and a capped read doesn't look like a complete history. Lite's Job History tab has the same 2,000-row cap and the same read-in-flight window, but neither cue.

What changes

• Lite/Controls/JobHistoryTab.xaml: adds a LoadingMessage text block, styled like the tab's existing empty-state message, collapsed by default.
• Lite/Controls/JobHistoryTab.xaml.cs:
  • a named RowCap = 2000 constant, replacing the literal 2000 passed to GetJobHistoryAsync;
  • LoadJobsAsync shows LoadingMessage before the read starts and collapses it in a finally, so it clears on every exit path (success, the caught exception, and the load-guard's superseded/early return);
  • the run-count label runs the unfiltered read's count (all.Count), not the filtered display count, through the shared PerformanceMonitor.Common.JobHistoryCap.Label (already referenced by Lite for AgTopology), appending (showing the newest 2,000) only when the read actually hit the cap.

Test plan

• New pin, Darling/Darling.Tests/LiteJobHistoryLoadingCapTests.cs: a source-scan fact (the same idiom as ViewerPerfmonShapingParityTests), since Lite's UserControl is WPF and cannot be instantiated off Windows. It asserts:
  • JobHistoryTab.xaml.cs calls JobHistoryCap.Label(, and its LoadJobsAsync sets LoadingMessage.Visibility = Visibility.Visible before the read and Visibility.Collapsed inside a finally;
  • JobHistoryTab.xaml declares x:Name="LoadingMessage".
  • RED on dev: both facts fail — dev's Lite tab has neither the call nor the element.
  • Mutation: dropping the JobHistoryCap.Label( call (temporarily replacing it with "") reproduces the RED on the call-site fact; reverted.
• Ran LiteJobHistoryLoadingCapTests + JobHistoryCapLabelTests (the label text itself, already pinned by The Viewer counts Availability Groups by identity, shows Job History loading, and says "queries" (#4475, #4478) #4481, not duplicated here) + DocCommentHygieneTests: Total 82, 0 failed, on macOS via the in-process xunit v3 runner.
• Built Darling.Tests.csproj and Lite.Tests.csproj Release with -p:EnableWindowsTargeting=true: 0 warnings, 0 errors on both. Lite's WPF UI (the XAML rendering, the loading/label wiring in a live grid) is exercised only in the Windows CI build; the pin here is source-level and cannot see a rendering bug that keeps the same source text.

CHANGELOG

SECTION: Fixed
ENTRY:

• Lite's Job History shows that it is loading instead of an empty grid, and says when it shows only the newest rows ([Lite's Job History shows that it is loading and says when it shows only the newest rows (#4478) #4488]).
  REF:
  [Lite's Job History shows that it is loading and says when it shows only the newest rows (#4478) #4488]: Lite's Job History shows that it is loading and says when it shows only the newest rows (#4478) #4488