fix(lsp): malformed edit if apply_text_edits() is called twice (#39347)

Problem:
Use vim.lsp.util.apply_text_edits to re-apply the same textedit causes
an incorrect edit, because apply_text_edits silently modifies the
parameter.

Solution:
- Avoid changing `text_edit._index`.
- Document this fun feature.

Helped-by: Riley Bruins <ribru17@hotmail.com>
Helped-by: Yi Ming <ofseed@foxmail.com>

(cherry picked from commit 790a8be5f3)

Co-authored-by: geril07 <62308020+geril07@users.noreply.github.com>
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

runtime/doc/lsp.txt

@@ -2870,7 +2870,8 @@ apply_text_document_edit({text_document_edit}, {index}, {position_encoding},
                                              *vim.lsp.util.apply_text_edits()*
 apply_text_edits({text_edits}, {bufnr}, {position_encoding},
                  {change_annotations})
-    Applies a list of text edits to a buffer.
+    Applies a list of text edits to a buffer. Note: this mutates `text_edits`
+    (sorts in-place and adds `_index` fields).
 
     Parameters: ~
       • {text_edits}          (`(lsp.TextEdit|lsp.AnnotatedTextEdit)[]`)

runtime/lua/vim/lsp/util.lua

@@ -291,7 +291,9 @@ local function get_line_byte_from_position(bufnr, position, position_encoding)
   return col
 end
 
---- Applies a list of text edits to a buffer.
+--- Applies a list of text edits to a buffer. Note: this mutates `text_edits` (sorts in-place and
+--- adds `_index` fields).
+---
 ---@param text_edits (lsp.TextEdit|lsp.AnnotatedTextEdit)[]
 ---@param bufnr integer Buffer id
 ---@param position_encoding 'utf-8'|'utf-16'|'utf-32'
@@ -320,7 +322,10 @@ function M.apply_text_edits(text_edits, bufnr, position_encoding, change_annotat
     -- Fix reversed range and indexing each text_edits
     for index, text_edit in ipairs(text_edits) do
       --- @cast text_edit lsp.TextEdit|{_index: integer}
-      text_edit._index = index
+      -- XXX: Preserve existing _index to avoid surprises if the same edit is reapplied. #39344
+      if text_edit._index == nil then
+        text_edit._index = index
+      end
 
       if
         text_edit.range.start.line > text_edit.range['end'].line