fix: route txt/md KB uploads to TextParser and clarify missing markitdown

[lxfight]

Fixes #9598

.txt / .md / .markdown uploads were routed to MarkitdownParser, which imports markitdown-no-magika at module level. On environments where that package is absent, uploading a txt file failed with a generic "文档解析失败：无法读取或解析上传文件" error that never mentioned the actual cause (ModuleNotFoundError). Meanwhile TextParser — a dependency-free plain-text parser explicitly written for TXT/MD — existed but was never used by select_parser.

Modifications / 改动点

• astrbot/core/knowledge_base/parsers/util.py:

  • Route .txt / .md / .markdown to TextParser (no external dependency needed; behavior is unchanged since markitdown just passed these through).
  • Keep .rst/.adoc/.xlsx/.docx/.xls on MarkitdownParser, but catch ImportError at selection time and raise KnowledgeBaseUploadError with an explicit install hint (pip install 'markitdown-no-magika[docx,xls,xlsx]'). kb_helper re-raises KnowledgeBaseUploadError untouched, so users now see the real cause instead of the misleading "file corrupted" message.

• tests/test_kb_select_parser.py: cover routing for all extensions, unsupported extension error, and the missing-dependency error path (simulated via sys.modules).

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

$ uv run pytest tests/test_kb_select_parser.py tests/test_epub_parser.py tests/test_kb_import.py -q
22 passed, 2 warnings in 2.97s

• uv run ruff format / uv run ruff check pass.
• Verification steps: in an environment without markitdown-no-magika, upload a .txt file to a knowledge base — it now parses and indexes successfully; uploading a .docx in the same environment reports the missing-dependency hint instead of a generic parse failure.

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Ensure knowledge-base uploads use the appropriate parser and expose actionable errors for missing optional dependencies.

Bug Fixes:

• Route plain-text knowledge-base uploads through the dependency-free TextParser so TXT and Markdown files continue to work when markitdown is unavailable.
• Report a clear installation hint when formats requiring markitdown-no-magika are uploaded without that dependency.

Tests:

• Add parser-selection coverage for text, office, EPUB, PDF, unsupported formats, and missing markitdown dependencies.