fix: support BOM-prefixed encodings in KB TextParser

[lxfight]

Fixes #9904

TextParser only tried ["utf-8", "gbk", "gb2312", "gb18030"], so txt files saved from Windows Notepad as "UTF-8 with BOM" or "Unicode" (UTF-16 LE) failed to decode and KB upload reported an unclear "无法读取或解析上传文件" error.

Modifications / 改动点

• astrbot/core/knowledge_base/parsers/text_parser.py: detect UTF-8 / UTF-16 LE|BE BOMs first and decode accordingly; BOM-less files keep the original utf-8 → gbk → gb2312 → gb18030 sequence.
• tests/test_text_parser.py: add unit tests covering plain UTF-8, UTF-8 BOM, UTF-16 LE/BE BOM, GBK, and undecodable input.

Note: BOM detection (instead of blindly adding utf-16 to the try sequence) is intentional — utf-16 decodes almost any byte stream, so trying it before GBK would silently mis-decode BOM-less GBK text into garbage instead of raising UnicodeDecodeError.

• 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_text_parser.py -q
......                                                                   [100%]
6 passed in 0.41s

• uv run ruff format / uv run ruff check pass.
• Verification steps: in Windows Notepad, save a txt via "另存为 → 编码: Unicode" (UTF-16 LE with BOM) and upload it to a knowledge base; it now ingests correctly instead of failing.

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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

Support BOM-prefixed text encodings in the knowledge-base text parser while retaining reliable fallback decoding for BOM-less files.

Bug Fixes:

• Enable knowledge-base text uploads to correctly decode UTF-8, UTF-16 LE, and UTF-16 BE files with byte-order marks.
• Preserve the existing encoding fallback behavior for BOM-less files and report undecodable content consistently.

Tests:

• Add coverage for UTF-8, BOM-prefixed UTF-8 and UTF-16, GBK, and undecodable text parsing.