Fix regex in mdextractor for code blocks with escaped backticks

[chigwell]

User Story
As a maintainer,
I want robust test coverage for code blocks containing escaped backticks
so that the Markdown parser reliably handles edge cases without false positives.

Background
The current regex pattern in mdextractor/__init__.py uses r"```(?:\w+\s+)?(.*?)```" with re.DOTALL, which may prematurely close code blocks containing legitimate backticks (e.g., echo "```"). This creates maintenance risks:

• The test_nested_code_blocks unit test demonstrates incorrect parsing of inner backticks
• Real-world code snippets with escaped backticks could be truncated
• Language specifier detection might interfere with content extraction

Acceptance Criteria

• Add test case to tests/test_mdextractor.py verifying:

  def test_backticks_inside_code_content(self):
      text = '''```sh
      echo "```"
      ```'''
      self.assertEqual(extract_md_blocks(text), ['echo "```"'])

• Update regex pattern to handle backticks within code content
• Ensure existing tests pass after modification
• Verify extraction works for:
  • Code blocks containing \``, ``, and ```` sequences
  • Mixed-language examples with internal backticks
  • Consecutive valid blocks separated by text
• Document pattern limitations in function docstring if any edge cases remain