[vim/vim] patch 9.2.0959: multiline dictionaries in :command and :autocmd cause E488 (PR #21073)

Problem:  A multiline dictionary argument to :command or :autocmd is parsed as a command block in Vim9 script.

Location: The issue is in the command-line collection path used by :command and :autocmd. In src/usercmd.c, may_get_cmd_block() relies on find_cmd_block_start() to determine whether subsequent lines belong to a command block. A trailing

{ in a multiline dictionary was not collected correctly, so the following dictionary entries were parsed as separate Ex commands and produced E488.
Solution: Extend the existing command-line collection logic to recognize a trailing dictionary opener when it follows an expression delimiter. Actual :command and :autocmd blocks continue to use find_cmd_block_start(), while commands such as normal! { remain ordinary command arguments and are not treated as blocks.
The fix uses the existing command-block collection path rather than introducing a separate parser or changing Vim9 expression parsing. This keeps the change localized to the existing :command and :autocmd handling and preserves the behavior added for nested command blocks.
Regression coverage was added for multiline dictionary arguments at script level and inside :def functions for both :command and :autocmd. Existing nested command-block and normal! { coverage also passes.
Tests:
- make -C src -j2
- make -C src/testdir test_usercommands.res
- make -C src/testdir test_autocmd.res
- make -C src/testdir test_vim9_script.res
- make -C src/testdir codestyle
- git diff --check
Fixes: #21070
The commit includes a Signed-off-by trailer and discloses AI assistance in the commit metadata.
You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/21073

-- Commit Summary --

  * patch 9.2.0959: multiline dictionaries are parsed as command blocks

-- File Changes --

    M src/testdir/test_autocmd.vim (21)
    M src/testdir/test_usercommands.vim (21)
    M src/usercmd.c (28)
    M src/version.c (2)

-- Patch Links --

https://github.com/vim/vim/pull/21073.patch
https://github.com/vim/vim/pull/21073.diff

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