GO-7038 Strip trailing newlines when pasting over selection at end of…

[KirillSto]

Summary

• Fix extra blank line appearing when pasting text over a selection that extends to the end of a code block
• Clipboard content typically includes a trailing \n which, when there's no text after the selection to absorb it, creates blank lines that accumulate with each paste
• Strip trailing newlines from paste text only when the selection reaches the end of the code block; mid-text pastes preserve trailing newlines as separators

Test plan

• Unit tests pass (TestClipboard_PasteToCodeBlock_TrailingNewline — 3 subtests)
• Existing TestClipboard_PasteToCodeBlock still passes (no regression)
• Full clipboard test suite passes
• Manual verification: paste over selection at end of code block, no extra blank line (QA is needed, as fix should be verified on Windows)

🤖 Generated with Claude Code

[github-actions]

New Coverage	46.5% of statements
Patch Coverage	100.0% of changed statements (3/3)

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