fix(terminal): interrupt/got_int hangs terminal (#30056)

Upon `terminal_enter`, `mapped_ctrl_c` is set in order to avoid `CTRL-C`
interrupts (which is proxied to the terminal process instead), `os_inchar`
will then test `mapped_ctrl_c` against `State` and set `ctrl_c_interrupts=false`
which prevents `process_ctrl_c` from setting `got_int=true` in a terminal
state.

However, if `got_int` is set outside of `process_ctrl_c`, e.g. via
`interrupt()`, this will hang the neovim process as `terminal_execute` will
enter an endless loop as `got_int` will never be cleared causing `safe_vgetc`
to always return `Ctrl_C`.

A minimal example reproducing this bug:
```vim
:autocmd TermEnter * call timer_start(500, {-> interrupt()})
:terminal
:startinsert
```

To fix, we make sure `got_int` is cleared inside `terminal_execute` when
it detects `Ctrl_C`.

Closes #20726

Co-authored-by: zeertzjq <zeertzjq@outlook.com>

src/nvim/terminal.c

@@ -782,6 +782,12 @@ static int terminal_execute(VimState *state, int key)
     FALLTHROUGH;
 
   default:
+    if (key == Ctrl_C) {
+      // terminal_enter() always sets `mapped_ctrl_c` to avoid `got_int`. 8eeda7169aa4
+      // But `got_int` may be set elsewhere, e.g. by interrupt() or an autocommand,
+      // so ensure that it is cleared.
+      got_int = false;
+    }
     if (key == Ctrl_BSL && !s->got_bsl) {
       s->got_bsl = true;
       break;

test/functional/terminal/buffer_spec.lua

@@ -312,6 +312,16 @@ describe(':terminal buffer', function()
       pcall_err(command, 'write test/functional/fixtures/tty-test.c')
     )
   end)
+
+  it('external interrupt (got_int) does not hang #20726', function()
+    eq({ mode = 't', blocking = false }, api.nvim_get_mode())
+    command('call timer_start(0, {-> interrupt()})')
+    feed('<Ignore>') -- Add input to separate two RPC requests
+    eq({ mode = 't', blocking = false }, api.nvim_get_mode())
+    feed([[<C-\><C-N>]])
+    eq({ mode = 'nt', blocking = false }, api.nvim_get_mode())
+    command('bd!')
+  end)
 end)
 
 describe(':terminal buffer', function()