Follow-ups from #364-#371: openFile with a pending operator, preview selection, selection edge cases #374

Verification of #364 to #371 (the fixes for #362) found these. Each one is on main before those PRs too, or is a narrow case the PRs did not claim to cover. Items 1 to 3 come first.

1. openFile with a pending operator: a deferred key reader can still let the operator act. fix(open_file): never change the buffer when openFile selects #370 cancels a pending operator (user typed d) by feeding <Esc>. If the opened buffer has a BufEnter callback that uses vim.schedule to read a key (getchar, confirm, a vim.ui prompt) or runs nvim_feedkeys(..., "x"), that reader eats the <Esc>. The user's next w then deletes from where they typed d to openFile's cursor (149 lines in the advisor's probe, Neovim 0.8.3 and 0.12.3). It needs that third-party hook shape.
2. openFile after A<C-o>d: the cancel returns to Insert mode, so the user's next key is inserted as text (one|wtwo|three|four).
3. openFile with preview: true puts the selection in the main window's buffer, not in the previewed file.
4. Smaller openFile gaps. The reply has no cancel note for a plain openFile (no range). An error after openFile moved windows leaves the operator pending. With makeFrontmost: false, a safe pending d is cancelled. On Neovim 0.12.3, openFile from Insert mode stays in Insert mode and selects nothing. On 0.8 to 0.10, a partly typed command line is lost. From the command-line window it fails with E11.
5. Selection tracking.
   • With 'selectmode' containing cmd, a Select-mode selection reaches Claude as an empty selection.
   • A selection that runs past the end of the line (v$) is sent without the line break Neovim yanks, with end = line length + 1.
   • On Neovim 0.11+, a combining mark right after a control character (TAB, NUL, ESC) makes the end one character different from Neovim's yank.
6. Column units. The plugin sends 0-based UTF-8 byte offsets. VS Code positions count UTF-16 code units, so the numbers differ on non-ASCII lines. Decide which one Claude expects.
7. filePath/fileUrl are not cleaned. A buffer name with invalid UTF-8 can still reach the client as invalid text (the selection text is cleaned since fix(selection): end charwise and blockwise selections on a whole character #367).
8. Neovim 0.8.3 core quirk. Reading vim.bo[other], nvim_get_option_value(..., {buf = other}) or nvim_buf_call(other, ...) while an exclusive Visual selection ends past the end of a line moves the end back one column. A third-party plugin that walks buffers during Visual mode would shrink the user's selection. Fixed in Neovim 0.9.5.
9. CI and test hygiene.
   • The integration-tests (v0.8.0) leg installs a dev build (v0.8.0-1210-gd367ed9b2), not the 0.8.0 release, and CI clones plenary.nvim at its latest commit.
   • fixtures/issue-238/README.md:44 still shows the nvim -l command, which hangs on 0.8.
   • Shuffled busted runs fail in mcp_tools_spec, init_spec and diff_ui_cleanup_spec (also without the new specs).

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