fix(zip): missed "password:" prompt if output trails it #41364

Problem:
Flaky test:

    RUN      T1158 nvim.zip reports an incorrect archive password: 11092.84 ms FAIL

The prompt is detected only if the pty output *ends* with "password: ",
but a read may return the prompt plus following bytes.

Solution:
- Match anywhere in the output since the last password was sent. The
  buffer is cleared before each send, so won't match stale text.
- Assert on the reported message, so a failure shows what was reported.

runtime/lua/nvim/zip.lua

@@ -198,8 +198,8 @@ local function extract_with_password(command, source, path, dir)
 
   local function wanted()
     return exited ~= nil
-      or buffered:find('password: $') ~= nil
-      or buffered:find('reenter: $') ~= nil
+      or buffered:find('password: ', 1, true) ~= nil
+      or buffered:find('reenter: ', 1, true) ~= nil
   end
 
   local reenter = false

test/functional/plugin/zip_spec.lua

@@ -447,14 +447,16 @@ describe('nvim.zip', function()
     local reported = exec_lua(function(uri)
       vim.api.nvim_input('no1<CR>no2<CR>no3<CR>')
       vim.api.nvim_cmd({ cmd = 'edit', args = { uri }, magic = { file = false, bar = false } }, {})
-      return vim.wait(1000, function()
-        return vim.api
-          .nvim_exec2('messages', { output = true }).output
-          :find('incorrect password', 1, true) ~= nil
+      local function messages()
+        return vim.api.nvim_exec2('messages', { output = true }).output
+      end
+      vim.wait(1000, function()
+        return messages():find('zip: ', 1, true) ~= nil
       end)
+      return messages()
     end, ('zip://%s/secret.txt'):format(archive))
 
-    eq(true, reported)
+    t.matches('incorrect password', reported)
   end)
 
   describe('extract', function()