fix: add docstring to MCP tool /md endpoint

[majiayu000]

Summary

Fixes #1652

The get_markdown endpoint was missing a docstring, causing the MCP tool to have an empty description when listed via list_tools(). This made it difficult for LLMs to understand the tool's purpose.

List of files changed and why

• deploy/docker/server.py - Added docstring to get_markdown function explaining the supported extraction modes (fit, raw, bm25, llm) and when to use this tool
• tests/unit/test_mcp_tool_docstrings.py - Added unit test using AST parsing to verify MCP tool docstrings are present

How Has This Been Tested?

• Unit test verifies docstring exists and contains expected content
• Test uses AST parsing to avoid import dependency issues

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes