Fixes#222
Address issue with `markitdown.convert_stream` crashing on input with leading blank characters or line breaks.
* Modify `convert_stream` function in `src/markitdown/_markitdown.py` to strip leading blank characters or line breaks from the input stream using a new helper function `_strip_leading_blanks`.
* Add a test case in `tests/test_markitdown.py` to verify that `markitdown.convert_stream` handles input with leading blank characters or line breaks correctly.
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For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/microsoft/markitdown/issues/222?shareId=XXXX-XXXX-XXXX-XXXX).
* refactor(tests): simplify string validation in tests
Introduce a helper function `validate_strings` to streamline the
validation of expected and excluded strings in test cases. Replace
repetitive string assertions in the `test_markitdown_local` function
with calls to this new helper, improving code readability and
maintainability.
* run pre-commit
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Co-authored-by: lumin <71011125+l-melon@users.noreply.github.com>
Co-authored-by: gagb <gagb@users.noreply.github.com>
