TerminalControl: skip whitespace-only copy-on-select

[tianrking]

Summary of the Pull Request

Prevents copyOnSelect from replacing the clipboard when the selected payload is empty or contains only whitespace.

References and Relevant Issues

Closes #11751.

Detailed Description of the Pull Request / Additional comments

The automatic copy path now suppresses clipboard events when the extracted plain text consists entirely of whitespace. The suppression is opt-in at the internal copy helper and is enabled only for mouse-release copy-on-select, so explicit copy actions continue to copy whitespace.

The mouse selection is still marked as handled when an automatic copy is suppressed. This preserves the existing copy-on-select behavior where a subsequent right-click pastes instead of re-copying the whitespace selection.

Validation Steps Performed

• Added ControlInteractivity coverage for a trimmed empty selection, multiline whitespace, the subsequent right-click paste path, non-whitespace automatic copy, and explicit whitespace copy.
• clang-format --dry-run --Werror passed for all changed files.
• Call-site, CRLF, and git -c core.whitespace=cr-at-eol diff --check validation passed.
• The local Control.UnitTests.vcxproj build was blocked before compilation because the checkout did not contain required repository NuGet packages, including Microsoft.Windows.ImplementationLibrary.1.0.250325.1. Restore from the configured TerminalDependencies feed was unavailable in this environment, so the test DLL could not be produced locally.

PR Checklist

• Closes Add ability to not automatically copy whitespace-only selections #11751
• Tests added/passed
• Documentation updated
  • If checked, please file a pull request on our docs repo and link it here: #xxx
• Schema updated (if necessary)

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