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

0 files changed, 0 insertions, 0 deletions