Plan viewer: guard clipboard writes, and hand back the captured query on a truncated single-statement copy

[erikdarlingdata]

Fixes #4582. Part of #4511.

Why

erikdarlingdata/PerformanceStudio@7cd9218 hardened every viewer clipboard write against a busy
Windows clipboard: another process holding it briefly (a clipboard manager, Office, RDP) makes
SetTextAsync/SetText throw, and an unguarded call crashes the app. PM already had a guarded
clipboard READ (ClipboardText.TryRead, #2833) but no guarded write, and the plan viewer's Copy
Query Text / Copy Operator Name / Copy Predicate / Copy Seek Predicate entries, plus the blocking
chain, deadlock graph, and DataGrid export copy helpers, all called Clipboard.SetText /
Clipboard.SetDataObject unguarded.

Separately, erikdarlingdata/PerformanceStudio@35249e2 (corrected by @fdd3d22) fixed Copy Query Text
to fall back to the query a plan was captured from when the plan's own copy hit SQL Server's
4,000-character showplan cap — but only for single-statement plans, counted the way the Statements
grid counts them (flattened into stored procedure/UDF bodies), not Batches.Sum (which stops at
the outer batch and would mis-treat EXEC dbo.SomeProc as single-statement).

What changes

• PerformanceMonitor.Ui/ClipboardText.cs: new TrySetText(string), mirroring TryRead's
  bounded retry (8 attempts, 25 ms apart) over a swappable WriteOnce seam so a pin can force the
  clipboard-open failure without a real busy clipboard.
• Every Clipboard.SetText/SetDataObject call in PerformanceMonitor.Ui now routes through
  ClipboardText.TrySetText: PlanViewerControl.xaml.cs (Copy Query Text),
  PlanViewerControl.Interaction.cs (Copy Operator Name / Object Name / Predicate / Seek
  Predicate), BlockingChainControl.xaml.cs and DeadlockGraphControl.xaml.cs (Copy SQL Text),
  and DataGridExport.cs (Copy Cell / Copy Row / Copy All Rows — used by ~29 grids across Lite and
  the Darling Viewer).
• PerformanceMonitor.PlanAnalysis/PlanDisplayText.CopyQueryText(statement, statementCount, capturedQueryText): the pure fallback rule. Returns the captured text only when the statement
  is truncated (IsTextTruncated), the plan is single-statement by the caller's already-flattened
  count, and captured text is available; otherwise the plan's own text.
• PlanViewerControl: added CapturedQueryText (set by LoadPlan's existing queryText
  parameter — no host wiring needed, both hosts already pass it where they have it) and
  _allStatementsCount (the same flattened count PopulateStatementsGrid already builds), and
  wired CopyStatementText_Click through the new helper.

Lite: gets this too (shared control)

Yes — PlanViewerControl, DataGridExport, and both context-menu controls live in
PerformanceMonitor.Ui and are shared by Lite and the Darling Viewer. Both hosts' plan-opening call
sites (Lite/Controls/ServerTab.Plans.cs, Darling/PerformanceMonitor.Darling.Viewer/ViewerServerTab.Plans.cs)
already pass a queryText argument to LoadPlan at most call sites (Query Store stored plans,
active-query snapshots), so CapturedQueryText is populated there for free. Call sites that open a
plan with no stored query text (e.g. a plan opened from a saved .sqlplan file, or a cache-fetched
plan with no query text available) pass null, and the fallback simply doesn't fire for those — the
plan's own (possibly truncated) text is returned, same as today, with Rule 39 still flagging the
truncation.

Not ported / follow-up

• Lite/Helpers/ContextMenuHelper.cs, Lite/Controls/ServerTab.CopyExport.cs,
  Lite/Controls/RecommendationsTab.xaml.cs, Lite/Windows/SettingsWindow.xaml.cs,
  Lite/Windows/AlertDetailWindow.xaml.cs, Lite/Windows/EntraDeviceCodeWindow.xaml.cs,
  Darling/PerformanceMonitor.Darling.Viewer/MainWindow.xaml.cs,
  ViewerServerTab.CopyExport.cs, ViewerServerTab.ChartContextMenu.cs, SettingsWindow.xaml.cs,
  AlertDetailWindow.xaml.cs (18 unguarded Clipboard.SetText/SetDataObject call sites total)
  have the same unguarded shape but sit outside the shared PerformanceMonitor.Ui plan-viewer
  surface this issue scoped to. They're filed as Lite and the Darling Viewer: 18 unguarded clipboard writes can crash the app when the clipboard is busy #4594.
• deprecated/Dashboard/** gets no work per standing policy.
• PS's CopyParameterizedStatementText_Click fallback exclusion (@fdd3d22's other half) has no PM
  equivalent yet — PM has no parameterized-copy menu entry to guard.

Test plan

New pure pins in Darling/Darling.Tests/Viewer4582Tests.cs (macOS-runnable, no WPF):

• single-statement truncated + captured text → returns captured text
• single-statement not truncated → returns the plan's own text even with captured text available
• truncated + no captured text → returns the plan's own (truncated) text
• multi-statement by grid count (one batch statement, statementCount: 2) truncated → returns the
  plan's own text, NOT the captured outer batch (the @fdd3d22 bug pinned directly)

RED: on origin/dev (461d503), the same test file fails to compile —
PlanDisplayText has no CopyQueryText member (CS0117, all 4 tests). Confirmed via a detached
worktree at that commit.

Mutation: flipping statementCount == 1 to statementCount >= 1 in CopyQueryText turns the
multi-statement pin (case 4 above) [FAIL]; reverted, and a clean rebuild confirmed 4/4 GREEN
again.

Run (macOS, in-process, Microsoft.WindowsDesktop.App stripped from the runtimeconfig):

Total: 235, Errors: 0, Failed: 0, Skipped: 2, Not Run: 0

(Viewer4582Tests plus the full required list: ShowPlanParserCondAndMultiplePlanTests,
ActualPlanRequestTests, ActualPlanDispatchTests, ActualPlanResultParseTests,
QueryModificationDetectorTests, ActualPlanCaptureLoopTests, ActualPlanGatingTests,
ReproScriptBuilderHardeningTests, DarlingAnalysisPipelineTests,
DarlingMcpPlanToolsSurfaceAndSqlTests, DarlingMcpPlanToolsLivePostgresTests,
McpPlanAnalysisEnvelopeTests, SerialLoopStoreSizeSourceTests, TsqlConventionGuardTests,
DocCommentHygieneTests.)

Builds, Release, -p:EnableWindowsTargeting=true, 0 warnings each: Darling.Tests,
PerformanceMonitor.PlanAnalysis, PerformanceMonitor.Ui, Lite/PerformanceMonitorLite.csproj,
Lite.Tests, Darling/PerformanceMonitor.Darling.Viewer.

Not run — Windows-only, CI decides them: Darling.Tests and Lite.Tests themselves build on
macOS but their WPF-dependent Viewer* classes can't discover/run here for PresentationFramework
reasons; none of them reference the files this PR touches, so they're unaffected. Lite.Tests has
no new/changed test in this PR (the guarded-write pin lives in Darling.Tests, which does run
in-process on macOS).

Screenshot plan (for a human on Windows, since the WPF wiring itself has no macOS-runnable
pin): open a plan with a single statement whose text is at or beyond the showplan cap (any plan
whose StatementText is ≥3,990 characters) via a Query Store "View Plan" path that passes stored
query text (so a full, untruncated query is available); right-click the row in the Statements
panel and choose "Copy Query Text"; paste — the pasted text should be the full stored query, not a
string ending mid-token. Then hold the Windows clipboard open in another app (e.g. an active
Office/Word paste-preview) and repeat any Copy action in the plan viewer, the blocking chain
control, or a DataGrid — none should crash; a transient failure should simply not copy.

CHANGELOG

SECTION: Fixed
ENTRY: - The plan viewer no longer crashes when another program is holding the clipboard, and "Copy Query Text" now hands back the full query on a truncated single-statement plan ([#4593]) - Copying anything from the plan viewer, blocking chain, deadlock graph, or a results grid used to crash the app if the Windows clipboard was briefly locked by another program; copies now fail quietly instead. Separately, "Copy Query Text" on a single-statement plan whose text hit SQL Server's 4,000-character cap now returns the full query the plan was captured from, instead of a copy cut off mid-word — for the Query Store and Active Queries plan sources that pass the original query text.
REF: [#4593]: #4593