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