[vim/vim] Vim9: E488 when :command or :autocmd argument ends with "{" of a multi-line dict (after 9.2.0908) (Issue #21070)

kechako created an issue (vim/vim#21070)

### Steps to reproduce

1. Save the following as `repro.vim`:

   ```vim
   vim9script
   def Run(): void
     command! -nargs=0 Foo call Bar('x', {
       'key': 'value',
     })
   enddef
   Run()
   echo 'OK'
   ```

2. Run `vim -N -u NONE -S repro.vim`

3. `E488: Trailing characters: : 'value',` is reported instead of `OK`.

4. The same happens with `:autocmd` in place of `:command`, and at Vim9 script level as well as inside a `:def` function:

   ```vim
   vim9script
   autocmd BufWritePre *.go call Bar('x', {
     'key': 'value',
   })
   ```

   These variants are **not** affected — the dict closed on the same line, and legacy script with line continuation:

   ```vim
   command! -nargs=0 Foo call Bar('x', {'key': 'value'})
   ```

   ```vim
   " legacy script
   command! -nargs=0 Foo call Bar('x', {
     \ 'key': 'value',
     \ })
   ```

### Expected behaviour

`OK` is echoed and the user command is defined.

This works as expected in v9.2.0907, and fails from v9.2.0908 through v9.2.0958, including the current master (`4e0cd139c`).


### Version of Vim

9.2.0958

### Environment

Operating system: macOS 26.5.2 (arm64)
Terminal: Ghostty
Value of $TERM: xterm-ghostty
Shell: zsh 5.9

### Logs and stack traces

```shell

```

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