fix: prevent KeyError on TextArea theme initialization (#6528)

[marcos-dataops]

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Closes #6528

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Hey folks! Ran into a crash when throwing a TextArea inside a ModalScreen. It throws a KeyError looking for text-area--gutter in the component styles because it tries to render before the DOM has fully resolved the initial component classes.

To fix this without breaking the render cycle, I just wrapped the get_component_rich_style call inside apply_css with a try/except. If the key isn't there yet, it safely falls back to None and continues smoothly.

I also dropped a quick regression test in test_issue_6528.py to make sure we don't catch this crash again in the future.

Let me know if you need any adjustments to this approach. Thanks!