What does this PR do?

$mju / $mjv parse the image index with a bare int() (plugins/linkai/midjourney.py:161). The two mistakes around that argument the code already anticipates -- one missing (命令缺少参数, line 158) and a number outside 1..4 (图片序号 N 错误, line 163) -- both answer the user, but a token that is not a number at all raises ValueError out of process_mj_task. It escapes on_handle_context into ChatChannel._fail_callback (channel/chat_channel.py:452-453), which only logs:

logger.exception("Worker return exception: {}".format(exception))

So $mju 11055927171882 2x produces no message of any kind -- not an error, not a hint about the expected shape. Copying an index out of a UI or typing 2x is an ordinary slip.

This PR guards the parse and replies through the same _set_reply_text path the neighbouring guards use, so the failure is visible to the user:

try:
    index = int(clist[1])
except ValueError:
    self._set_reply_text(f"图片序号 {clist[1]} 错误，应为 1 至 4 的数字", e_context)
    return

The three f prefixes removed elsewhere in the file carry no placeholders. The lint job runs ruff check --select F over the files a PR touches, so that inherited F-class debt in the same file would otherwise turn this PR's gate red.

Type of change

• Bug fix
• New feature
• Refactor / chore

Tests

tests/test_linkai_midjourney_index_argument.py drives process_mj_task(TaskType.UPSCALE, ...) through an EventContext, the way the existing tests/test_linkai_midjourney_send.py and tests/test_keyword_attachment_urls.py do, and asserts both that a non-numeric token now gets a reply and that the existing 1..4 message still fires for 9.

• Before the fix, on plain c7ae3ba5: 1 failed, 1 passed -- ValueError: invalid literal for int() with base 10: '2x' at plugins/linkai/midjourney.py:161.
• After the fix: 2 passed.
• pytest tests/ -q: 38 failed, 2216 passed on this branch vs 38 failed, 2214 passed on plain c7ae3ba5 -- same failures, +2 from the new file.
• ruff check plugins/linkai/midjourney.py tests/test_linkai_midjourney_index_argument.py --select F --ignore F403,F405: All checks passed.

Checklist

• I have read the Contributing Guide
• I tested this change locally
• Code comments and docs are in English
• Linked related issue: closes C:/Users/20235/.qoder/bin/git/$mjv with a non-numeric image index raises ValueError and the user gets no reply #3290