Fix KeyError and color blending crash on custom TextAreas

[amanparab1]

This Pull Request resolves two crashes associated with the TextArea widget:

1. KeyError in apply_css: Mounting or rendering a custom TextArea subclass with overridden or empty COMPONENT_CLASSES raised KeyError: "No 'text-area--gutter' key in COMPONENT_CLASSES". This happened because TextAreaTheme.apply_css called get_component_rich_style() directly without safety checks. This is resolved by wrapping style retrieval in a try-except block to return None on missing classes rather than crashing.
2. AttributeError in color blending: In the selection style fallback code, app_theme.primary (a string) was passed directly to background_color.blend(), which expects a Color object, raising AttributeError: 'str' object has no attribute 'auto'. This is fixed by wrapping the string in Color.parse().

Fixes

Fixes #6528

Verification

Added a unit test under tests/text_area/test_issue_6528.py that verifies TextArea subclassing with overridden component classes mounts and resolves styles correctly without crashes.