Make the Unix uv_tty_set_mode() implementation return UV_EINVAL for unrecognized uv_tty_mode_t values instead of reaching UNREACHABLE().

The Windows implementation already returns UV_EINVAL for invalid TTY modes, so this makes the behavior consistent across platforms and avoids treating caller-provided enum values as unreachable state.

Validation is performed before raw-mode normalization so invalid modes do not touch terminal state.

Refs: nodejs/node#64140

Local verification:

cmake -S . -B build -DBUILD_TESTING=ON
cmake --build build --target uv_run_tests -j4
./build/uv_run_tests tty_pty

tty_pty passed locally. tty skipped locally because this environment has no controlling /dev/tty.