fix: add missing KB parser routes for mdx/mkd/html/htm/csv

[lxfight]

Fixes #9903

select_parser and the MarkdownChunker whitelist in kb_helper.py were inconsistent:

• .mdx / .mkd are accepted by the chunker whitelist but had no parser route — uploads of these files always failed at the parsing stage with "暂时不支持的文件格式", making that chunker branch dead code.
• .html / .htm / .csv had no route at all, although the bundled markitdown-no-magika version registers HtmlConverter and CsvConverter, so the capability was wasted.

Modifications / 改动点

• astrbot/core/knowledge_base/parsers/util.py:
  • Route .mdx / .mkd to TextParser (plain markdown variants, no extra dependency; the chunker whitelist already expects them).
  • Route .html / .htm / .csv to MarkitdownParser.
• tests/test_kb_parser_routing.py: cover the new routes, a real HTML/CSV parse smoke test, and that .rtf stays unsupported.

Note on .rtf: the issue suggested routing it to MarkitdownParser, but markitdown-no-magika 0.1.2 registers no RTF converter (verified: md.convert(...) raises UnsupportedFormatException for .rtf), so adding the route would only move the failure to a later stage with a worse error message. It intentionally keeps raising the explicit "unsupported format" error. Content sniffing for extension validation (optional in the issue) is left out to keep this change minimal.

• 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_parser_routing.py tests/test_epub_parser.py tests/test_kb_import.py -q
18 passed, 2 warnings in 2.88s

• uv run ruff format / uv run ruff check pass.
• Verification steps: upload an .mdx/.mkd/.html/.htm/.csv file to a knowledge base — previously .mdx/.mkd failed with "暂时不支持的文件格式" and html/csv were rejected outright; now all of them parse and index.

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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

Add missing knowledge-base parser routes for supported document extensions while preserving explicit handling for unsupported RTF files.

New Features:

• Add parser support for MDX and MKD files.
• Add HTML, HTM, and CSV parsing through the existing MarkItDown integration.

Bug Fixes:

• Fix knowledge-base uploads failing for extensions already accepted by the Markdown chunker but lacking parser routes.

Tests:

• Add parser-routing coverage, HTML and CSV parsing smoke tests, and regression coverage confirming RTF remains unsupported.