fix(markdownparser): ensure merged filenames unique

[sponge225]

Description

导入 Markdown（ ov add-resource ）时，若文档中存在大量重复同名标题（例如重复的顶级 # ... ）， MarkdownParser 会把多个“小节”合并输出为 *_Nmore.md

旧的合并命名策略在 count > 1 时仅使用 {first}_{count}more ，在“重复标题 + 多次 merge 批次”场景下会产生同名文件，导致解析阶段写入覆盖/内容丢失

Related Issue

Closes #1004

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

仅调整合并文件命名逻辑： count > 1 时在 {first}_{count}more 后追加基于合并内容的短 hash（8 位），生成 {first}_{count}more_{hash8} ，确保不同合并批次的输出文件名稳定且唯一。

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

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)

Additional Notes

[CLAassistant]

All committers have signed the CLA.

[github-actions]

Failed to generate code suggestions for PR

[qin-ctx]

solve by #1005