Add flush helper to ClipboardCore

[JeremyKuhne]

Adds a flush helper (this is a public API in WPF). Tweaks some code in WinFormsOleServices for better clarity and error state handling.

Also adds a helper to check if a given object is on the clipboard.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

PR Overview

This PR adds two new helper APIs to improve clipboard data handling: a new Flush method that flushes clipboard data and an IsObjectOnClipboard method that validates whether a given object is currently on the clipboard. It also updates TryGetData's accessibility and improves error-handling clarity in WinFormsOleServices.

• Added Flush helper in ClipboardCore.cs.
• Introduced IsObjectOnClipboard to check clipboard object equality.
• Updated QueryGetData/GetData error handling in WinFormsOleServices.cs for better clarity.

Reviewed Changes

File	Description
src/System.Private.Windows.Core/src/System/Private/Windows/Ole/ClipboardCore.cs	Added Flush helper and IsObjectOnClipboard including retry logic
src/System.Windows.Forms/src/System/Windows/Forms/OLE/WinFormsOleServices.cs	Refactored error handling in GetData to separate query and retrieval steps

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

src/System.Windows.Forms/src/System/Windows/Forms/OLE/WinFormsOleServices.cs:94

• [nitpick] The variable 'result' is reused for both QueryGetData and GetData calls, which may reduce readability. Consider using a separate variable for the GetData call to enhance clarity.

result = dataObject->GetData(formatEtc, out STGMEDIUM medium);

[JeremyKuhne]

InvokeItemsDialog_ShouldThrowException test took down the run because it hit an assert for a negative test.

Process terminated. Assertion failed.
Need PropertyDescriptor for ICollection property to associate collection editor with.
   at System.Windows.Forms.Design.EditorServiceContext..ctor(ComponentDesigner designer, PropertyDescriptor prop) in /_/src/System.Windows.Forms.Design/src/System/Windows/Forms/Design/EditorServiceContext.cs:line 34
   at System.Windows.Forms.Design.EditorServiceContext.EditValue(ComponentDesigner designer, Object objectToChange, String propName) in /_/src/System.Windows.Forms.Design/src/System/Windows/Forms/Design/EditorServiceContext.cs:line 48
   at System.Windows.Forms.Design.ListControlUnboundActionList.InvokeItemsDialog() in /_/src/System.Windows.Forms.Design/src/System/Windows/Forms/Design/ListControlUnboundActionList.cs:line 20
   at FluentAssertions.Specialized.ActionAssertions.InvokeSubject()
   at FluentAssertions.Specialized.DelegateAssertions`2.InvokeSubjectWithInterception()
   at FluentAssertions.Specialized.DelegateAssertions`2.Throw[TException](String because, Object[] becauseArgs)
   at System.Windows.Forms.Design.Tests.ListControlUnboundActionListTests.InvokeItemsDialog_ShouldThrowException() in /_/src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/ListControlUnboundActionListTests.cs:line 49

[codecov]

Codecov Report

Attention: Patch coverage is 21.87500% with 25 lines in your changes missing coverage. Please review.

Project coverage is 76.10266%. Comparing base (342d09f) to head (d3a2071).
Report is 1 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13049         +/-   ##
===================================================
- Coverage   76.10589%   76.10266%   -0.00324%     
===================================================
  Files           3275        3275                 
  Lines         643778      643804         +26     
  Branches       47441       47445          +4     
===================================================
- Hits          489953      489952          -1     
- Misses        150263      150291         +28     
+ Partials        3562        3561          -1     

Flag	Coverage Δ
Debug	76.10266% <21.87500%> (-0.00324%)	⬇️
integration	18.04771% <0.00000%> (-0.00130%)	⬇️
production	50.13046% <21.87500%> (-0.00490%)	⬇️
test	96.95044% <ø> (ø)
unit	47.56166% <21.87500%> (-0.00640%)	⬇️

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