tui: libtermkey: force CSI driver for mouse input #7948

Fixes #7932

Nvim (tui.c) always enables SGR mouse (TUIData.unibi_ext.enable_mouse).
But if libtermkey sees key_mouse (kmous) in terminfo its terminfo driver
(driver-ti.c) will be activated, which by accident only supports X10
protocol. The libtermkey CSI driver (driver-csi.c), in contrast,
supports SGR.

We can force libtermkey to ignore the terminfo key_mouse entry by
returning NULL in the tui_tk_ti_getstr hook. That forces the CSI driver.

What is the effect of returning NULL from `tui_tk_ti_getstr()`?
- libtermkey `driver-ti.c:load_terminfo()` skips the entry.
- `termkey.c:peekkey()` iterates through all drivers, it finds
  `TERMKEY_RES_NONE` for the ti driver and falls back to the CSI driver.

src/nvim/tui/tui.c

@@ -1813,6 +1813,12 @@ static const char *tui_tk_ti_getstr(const char *name, const char *value,
     if (value != NULL && strequal(stty_erase, value)) {
       return stty_erase[0] == DEL ? CTRL_H_STR : DEL_STR;
     }
+  } else if (strequal(name, "key_mouse")) {
+    DLOG("libtermkey:kmous=%s", value);
+    // If key_mouse is found, libtermkey uses its terminfo driver (driver-ti.c)
+    // for mouse input, which by accident only supports X10 protocol.
+    // Force libtermkey to fallback to its CSI driver (driver-csi.c). #7948
+    return NULL;
   }
 
   return value;