refactor(parse): remove MinerU PDF strategy

[qin-ctx]

Description

OpenViking 当前同时保留了本地 pdfplumber 解析和 MinerU 远程转换。默认的 auto 模式先执行本地解析，失败后才可能调用 MinerU。仓库内没有为这条远程路径提供稳定的集成入口或契约覆盖，却额外引入了一套配置、回退逻辑和返回元数据。

修改前：

• PDFConfig.strategy 支持 auto / local / mineru。
• 本地解析失败时，auto 可根据 mineru_endpoint 回退到 MinerU。
• 解析结果会输出 pdf_strategy 和 strategy 元数据。

修改后：

• PDF 始终在本地使用 pdfplumber 提取文本、表格、图片和文档结构，再交给 MarkdownParser 构建上下文树。
• 同步 PDF 转换仍在线程池中执行，不阻塞事件循环。
• MinerU 实现、自动回退、配置、示例和策略元数据全部删除。

Human Involvement

• A human participated in the implementation or review loop
• This PR was generated entirely by AI agents without human participation in the loop

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• 删除 PDFParser 中的 MinerU 远程转换和 auto 回退分支，收敛为单一本地解析链路。
• 删除 strategy 和全部 mineru_* PDF 配置，更新中英文文档和示例配置。
• 更新现有 PDF 测试以调用唯一的转换入口；没有新增测试文件、测试函数或参数化用例。

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

66 passed:

tests/parse/test_pdf_bookmark_extraction.py
tests/parse/test_markdown_no_split.py
tests/parse/test_parser_config_wiring.py
tests/test_config_loader.py

同时通过 Ruff、git diff --check 和无新增测试用例检查。

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

N/A

Additional Notes

这是一个配置破坏性变更。升级后需从 parsers.pdf 中删除以下字段：

{
  "strategy": "auto",
  "mineru_endpoint": "...",
  "mineru_api_key": "...",
  "mineru_timeout": 300,
  "mineru_params": {}
}

这些字段不再被接受，配置加载时会明确报 Unknown config field。