feat(tui): expand collapsed paste placeholder when pasting again - #51766

What does this PR do?

Closes #8501.

Pasting again while the cursor sits on a collapsed paste placeholder ([Pasted ~N lines]) expands it back into the original text instead of inserting new clipboard content. This mirrors the behavior users know from other CLI agents: paste collapses, pressing paste again expands.

Demo

Buffer: check [Pasted ~3 lines] (cursor right after the placeholder) -> Cmd+V -> check alpha\nbeta\ngamma

Why this design

• No new keybind: reuses the existing prompt.paste command and the bracketed-paste path, so the same shortcut that created the placeholder toggles it back. This keeps keybinds clean (a concern raised on feat(tui): Allow to expand pasted text (AKA "pasted summary") #8496) and gives the interaction for free on both paths.
• Works for both entry points: explicit prompt.paste dispatch (clipboard read) and terminal bracketed paste (onPaste), which is what Cmd+V produces in iTerm/Terminal.
• Cursor-adjacent detection: expansion triggers when the cursor is on the placeholder range or right after it (the trailing space pasteText inserts). Moving the cursor elsewhere and pasting inserts clipboard content as before.
• Text parts only: file/image/agent extmarks sharing the same prompt-part typeId are ignored, so pasting over an [Image 1] badge still pastes.

Implementation

• packages/tui/src/prompt/part.ts: adds pastedTextExtmarkAtOffset (cursor-on-placeholder check incl. the trailing space) and expandPastedTextPlaceholder (finds the text-part extmark at the cursor, deletes the placeholder range, re-inserts the stored text in place, keeps remaining extmarks adjusted by the buffer's own hooks).
• packages/tui/src/component/prompt/index.tsx: calls it at the top of the prompt.paste command and the onPaste handler; on expansion it re-syncs prompt parts so the removed text part is dropped and other parts keep their ranges.

How did you verify your code works?

• New unit tests in packages/tui/test/prompt/paste-expand.test.tsx: offset matching, single expansion with cursor position check, no-op second paste, expanding only the placeholder under the cursor when several exist, and ignoring non-text extmarks (e.g. file badges).
• bun test in packages/tui: 199 tests pass, bun typecheck clean.