fix(windows): route Windows Terminal through the VT input path (#5335)

Windows Terminal does not set TERM by default, so zellij on WT was
using the native-console crossterm path that reads INPUT_RECORDs.
That path can't observe bracketed-paste sequences (`\e[200~/\e[201~`),
which is one of the reasons multi-line paste has never worked
correctly on Windows (issue #4885).

WT sets `WT_SESSION` per-process. Use that as a second trigger for
the VT input path so default WT installs get raw VT byte input (and
with it, termwiz's bracketed-paste detection, the SGR mouse format,
correct emoji delivery, etc.) without requiring the user to set TERM.

Introduce `use_vt_path()` in `os_input_output_windows.rs` as the
single source of truth for the gate. Previously the stdin handler
had one TERM check and `enable_mouse_support` / `disable_mouse_support`
each had their own — so adding `WT_SESSION` to just the stdin gate
caused mouse setup to take the native-console branch, which calls
`crossterm::event::EnableMouseCapture` and clobbers the
`ENABLE_VIRTUAL_TERMINAL_INPUT` flag the stdin path had just set.
That broke all input from WT until the helper aligned every site.

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