fix: avoid duplicate quoted image captions

[he-yufeng]

修复 #8708。

当引用消息里包含图片、且主对话模型不支持图片输入时，构建请求阶段会先把引用图片加入 req.image_urls，随后 _ensure_img_caption() 会统一生成 <image_caption>。之后 _process_quote_message() 又会重新处理同一张引用图片，并生成 [Image Caption in quoted message]，导致同一图片被转述两次、也多调用一次图转述模型。

这个改动在已经生成 <image_caption> 的请求里跳过引用图片的第二次转述，只保留 <Quoted Message> 里的引用文本和附件上下文。主模型支持图片输入时的现有跳过逻辑保持不变；没有生成全局图片转述时，引用图片转述 fallback 仍然可用。

验证：

• python -m py_compile astrbot\core\astr_main_agent.py tests\unit\test_astr_main_agent.py
• python -m ruff check astrbot\core\astr_main_agent.py tests\unit\test_astr_main_agent.py
• isolated pytest runner: tests/unit/test_astr_main_agent.py -q → 88 passed

说明：当前 Python 环境里有另一个 editable checkout 把 AnyCoder/tests 注册成了 tests 包，直接 pytest 会把 AstrBot 的 tests.fixtures 解析到错误位置；因此本地验证使用了隔离 runner，手动把本仓 tests 绑定到当前 worktree，并显式加载 pytest_asyncio.plugin。

Summary by Sourcery

Prevent duplicate caption generation for quoted images when a global image caption has already been created in main agent requests.

Bug Fixes:

• Ensure quoted images are not captioned twice in a single request when global image captioning has already handled them.

Tests:

• Add a regression test verifying that quoted images are only captioned once and that the quoted-message placeholder caption is omitted when a global caption exists.

[gemini-code-assist]

Code Review

This pull request introduces a mechanism to prevent duplicate captioning of quoted images. It adds a skip_quote_image_caption flag to _process_quote_message and determines if quote images have already been captioned by checking for the presence of <image_caption> tags in the request's extra user content parts. A corresponding unit test has been added to verify this behavior. There are no review comments, and I have no feedback to provide.

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[Dt8333]

这里的quote_images_already_captioned和下面的else处的应当可以提到if前。

[Dt8333]

是否可以考虑下image_seg统一判断一下。
目前4个if挨个判断有点...