Fix issue #1423: Update test input for Black 25.9.0

[rylena]

This PR fixes issue #1423 where test_codemod_formatter_error_input failed with black 25.9.0.
The issue was that black 25.9.0 no longer errors on the invalid syntax used in the test input file.
I updated the test input to use a match statement, which black (targeting Python 3.6) correctly rejects as a syntax error, while LibCST (configured for 3.6) parses it.
I also updated the expected error message in test_codemod_cli.py.

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