fix(terminal): OSC 52 multiline copy replaces newlines with NUL #41097

Problem: An OSC 52 sequence from a :terminal job passes the decoded
payload to the clipboard provider as a single list item. Command-line
providers (pbcopy, xclip, ...) receive it with channel semantics, where
a newline inside an item is sent as NUL (:h chansend()), so multiline
copies arrive with NUL bytes instead of newlines.

Solution: Split the payload on newlines into a proper list of lines.
A trailing newline yields a final empty item, which chansend() turns
back into a newline, so payloads round-trip exactly.

(cherry picked from commit e58f29ca3e)

test/functional/terminal/clipboard_spec.lua

@@ -18,6 +18,7 @@ describe(':terminal', function()
       local function clipboard(reg, type)
         if type == 'copy' then
           return function(lines)
+            vim.g.clipboard_lines = lines
             local data = table.concat(lines, '\n')
             vim.g.clipboard_data = data
           end
@@ -46,20 +47,40 @@ describe(':terminal', function()
     ]])
   end)
 
+  local function osc52(arg)
+    return string.format('\027]52;;%s\027\\', arg)
+  end
+
   it('can write to the system clipboard', function()
     eq('Test', eval('g:clipboard.name'))
 
     local text = 'Hello, world! This is some\nexample text\nthat spans multiple\nlines'
     local encoded = exec_lua('return vim.base64.encode(...)', text)
 
-    local function osc52(arg)
-      return string.format('\027]52;;%s\027\\', arg)
-    end
-
     fn.jobstart({ testprg('shell-test'), '-t', osc52(encoded) }, { term = true })
 
     retry(nil, 1000, function()
       eq(text, exec_lua([[ return vim.g.clipboard_data ]]))
     end)
+
+    -- Multiline payloads must arrive split into separate list items: a newline
+    -- inside a single item would be sent to a command-line provider as NUL.
+    eq({
+      'Hello, world! This is some',
+      'example text',
+      'that spans multiple',
+      'lines',
+    }, exec_lua([[ return vim.g.clipboard_lines ]]))
+  end)
+
+  it('multiline copy does not send newlines as NUL #41097', function()
+    local text = '\nfoo\n\nbar\n'
+    local encoded = exec_lua('return vim.base64.encode(...)', text)
+
+    fn.jobstart({ testprg('shell-test'), '-t', osc52(encoded) }, { term = true })
+
+    retry(nil, 1000, function()
+      eq({ '', 'foo', '', 'bar', '' }, exec_lua([[ return vim.g.clipboard_lines ]]))
+    end)
   end)
 end)