Avoid blocking the event loop in markitdown-mcp

[lavish0000]

Closes #1517

Summary:

• run MarkItDown.convert_uri() in a worker thread inside the async MCP tool
• keep the tool response shape unchanged while avoiding synchronous blocking on the event loop thread
• add a regression test that asserts the conversion work runs off the event loop thread

Testing:

• uv run --project packages/markitdown-mcp --with pytest,mypy python -m pytest packages/markitdown-mcp/tests/test_main.py
• uv run --project packages/markitdown-mcp --with mypy python -m mypy packages/markitdown-mcp/src/markitdown_mcp/__main__.py
• uv run --project packages/markitdown-mcp --with ruff python -m ruff check packages/markitdown-mcp/src/markitdown_mcp/__main__.py packages/markitdown-mcp/tests/test_main.py
• uv run --project packages/markitdown-mcp --with ruff python -m ruff format --check packages/markitdown-mcp/src/markitdown_mcp/__main__.py packages/markitdown-mcp/tests/test_main.py

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