highlighter: Preserve text color without syntax highlighting

[MuNeNiCK]

Related to #2325

Description

When the tree-sitter feature is disabled, the stub implementation of SyntaxHighlighter::styles returns an empty style list.

The input renderer builds text runs from that list. When the list is empty, no colored text run is created, so GPUI paints the text with its default black instead of preserving the input's text color.

The native highlighter returns the requested range with HighlightStyle::default() when no styles match. This change aligns the stub implementation with that behavior so the input text color is preserved when syntax highlighting is disabled.

How to Test

• Run cargo check -p gpui-component --target wasm32-unknown-unknown --lib.
• Build a web application that uses gpui-component with default-features = false.
• Enter text in a CodeEditor with a dark theme.
• Verify that existing and newly entered text use the input's text color.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (if any) is accurate.
• [N/A] Passed cargo run for story tests related to the changes.
• [N/A] Tested macOS, Windows and Linux platforms performance (not platform-specific).

[MuNeNiCK]

Thanks for merging the PR. I really appreciate it.