test: improve Nvim EOF handling (for Windows CI) #40841

Problem:
Mysterious Windows CI failure points to resource exhaustion, but we have
no visibility/instrumentation:

    RUN      T610 nvim_set_keymap, nvim_del_keymap can set mappings with special characters, lhs: <S-Left>, rhs: <S-Left>: 93.70 ms OK
    RUN      T611 nvim_set_keymap, nvim_del_keymap can set mappings with special characters, lhs: <S-Left>, rhs: <F12><F2><Tab>: 49.48 ms FAIL
    …\testnvim.lua:144: EOF was received from Nvim. Likely the Nvim process crashed.
    stack traceback:
            D:/a/neovim/neovim/test/functional/testnvim.lua:144: in function 'nvim_set_keymap'
            …/api/keymap_spec.lua:769: in function <…/api/keymap_spec.lua:768>
    RUN      T612 nvim_set_keymap, nvim_del_keymap can set mappings with special characters, lhs: <S-Left>, rhs: <Space><Tab>: 18.49 ms FAIL
    …\testnvim.lua:144: EOF was received from Nvim. Likely the Nvim process crashed.
    stack traceback:
            D:/a/neovim/neovim/test/functional/testnvim.lua:144: in function 'nvim_set_keymap'
            …/api/keymap_spec.lua:769: in function <.../build/Xtest_xdg_api/test/functional/api/keymap_spec.lua:768>

Solution:
Append the child exit-code and signal to the "EOF … crashed" message.

Example:

    Nvim EOF (crash?) exit code: 42 (0x0000002A)

On the next failing Windows run, the first crash line will classify the failure:
- 0xC0000005 → access violation (nvim bug)
- 0xC0000374 → heap corruption (nvim bug)
- spawn/resource error or "clean" exit-code → system resource exhaustion

test/functional/testnvim.lua

@@ -9,7 +9,6 @@ local SocketStream = uv_stream.SocketStream
 local ProcStream = uv_stream.ProcStream
 
 local check_cores = t.check_cores
-local pcall_err = t.pcall_err
 local check_logs = t.check_logs
 local dedent = t.dedent
 local eq = t.eq
@@ -133,7 +132,7 @@ end
 --- @return any
 function M.request(method, ...)
   assert(session, 'no Nvim session')
-  assert(not session.eof_err, 'sending request after EOF from Nvim')
+  assert(not session.eof_err, 'RPC request after Nvim EOF')
   local status, rv = session:request(method, ...)
   if not status then
     if loop_running then
@@ -331,12 +330,11 @@ end
 -- Use for commands which expect nvim to quit.
 -- The first argument can also be a timeout.
 function M.expect_exit(fn_or_timeout, ...)
-  local eof_err_msg = 'EOF was received from Nvim. Likely the Nvim process crashed.'
   if type(fn_or_timeout) == 'function' then
-    t.matches(vim.pesc(eof_err_msg), t.pcall_err(fn_or_timeout, ...))
+    t.matches(vim.pesc(Session.eof_err_msg), t.pcall_err(fn_or_timeout, ...))
   else
     t.matches(
-      vim.pesc(eof_err_msg),
+      vim.pesc(Session.eof_err_msg),
       t.pcall_err(function(timeout, fn, ...)
         fn(...)
         assert(session)