Problem:
Test sockets live under `$TMPDIR`, which the harness points at the build
dir.  On macOS/BSD `sockaddr_un.sun_path` is 104 bytes, and a CI build
path plus "nvim.<pid>.<n>" leaves little room:

    /Users/runner/work/neovim/neovim/build/Xtest_tmpdir_terminal/nvim.runner/aBcDeF/nvim.12345.0

Solution:
Point XDG_RUNTIME_DIR (`stdpath('run')`) at "/tmp/nvim_<pid>".  28 bytes:

    /tmp/nvim_19916/nvim.19919.1

TODO?:
- `TEMP_DIR_NAMES` prefers `$TMPDIR` over `/tmp`, so on macOS
  `stdpath('run')` defaults to the long `/var/folders/<xx>/<…>/T/` path
  instead of the short `/tmp` alias.

Note:
- The 104-byte limit applies to the `bind()` arg, not its "realpath",
  to, so `/tmp/…` symlinks can be used to workaround the limit.

This was initially added so that the __NVIM_DETACH environment variable
can be added to the jobs started in this test. That environment variable
is no longer needed, and there is also vim.tbl_extend() that can be used
to add an environment variable to a job anyway.

Also, make a shallow copy of opts.env in setup_child_nvim(), as mutating
the opts.env passed in may mask problems in other tests.

Problem:
When a terminal process exits, "[Process Exited]" text is added
to the buffer contents.

Solution:
- Return `exitcode` field from `nvim_get_chan_info`.
- Show it in the default 'statusline'.
- Show exitcode as virtual text in the terminal buffer.

fix: allocate hidden console for detached server

Starting the server with UV_PROCESS_DETACHED results in DETACHED_PROCESS, leaving the child without a console. Without a console:

CONIN$ / CONOUT$ cannot resolve, causing channel_from_stdio to fail.

ConPTY cannot attach, breaking :terminal.

This patch allocates a hidden console via AllocConsole() when the server has none, restoring working stdio and enabling ConPTY.

Also updates os_set_cloexec to clear HANDLE_FLAG_INHERIT on the RPC pipe
handles, matching the Unix F_DUPFD_CLOEXEC behavior.

TermCursor already has cterm=reverse. Additionally, now that terminal
buffers have a real cursor, the cterm=reverse in TermCursor no longer
shows up in the screen state.

Problem:
Every CI log has a lot of noise at the end, which makes it harder to
find relevant test failures:

    Running tests from test/functional/terminal/tui_spec.lua
    ...
    T5831 TUI bg color queries the terminal for background color:
    T5832 TUI bg color triggers OptionSet from automatic background processing:
    T5833 TUI bg color sends theme update notifications when background changes #31652:
    ...
    Running tests from test/functional/ui/output_spec.lua
    ...
    WRN 2025-12-02T03:36:47.304 ui/c/T5831.28003.0 tui_handle_term_mode:223: TUI: terminal mode 2026 unavailable, state 0
    WRN 2025-12-02T03:36:47.359 ui/c/T5832.28006.0 tui_handle_term_mode:223: TUI: terminal mode 2048 unavailable, state 0
    WRN 2025-12-02T03:36:47.414 ui/c/T5833.28009.0 tui_handle_term_mode:223: TUI: terminal mode 2048 unavailable, state 0

Solution:
- Skip logging in test-mode.
    - This can be reverted later, when these logs are changed to "INFO"
      level, per this TODO comment:
      ```
      // TODO(bfredl): This is really ILOG but we want it in all builds.
      // add to show_verbose_terminfo() without being too racy ????
      WLOG("TUI: terminal mode %d unavailable, state %d", mode, state);
      ```

Problem:
`termopen` has long been a superficial wrapper around `jobstart`, and
has no real purpose. Also, `vim.system` and `nvim_open_term` presumably
will replace all features of `jobstart` and `termopen`, so centralizing
the logic will help with that.

Solution:
- Introduce `eval/deprecated.c`, where all deprecated eval funcs will live.
- Introduce "term" flag of `jobstart`.
- Deprecate `termopen`.