refactor: optimize regex patterns for image and markdown processing

- Introduced pre-compiled regex patterns across various components, including streaming_helper, markdown_stream_formatter, and assistant_message_widget, to enhance performance during image extraction and markdown formatting.
- Updated the AssistantMessageWidget to utilize these optimized patterns for TTS sanitization and image processing, reducing unnecessary regex evaluations.
- Improved overall efficiency in handling markdown content by leveraging pre-compiled patterns for common markdown syntax detection.

lib/shared/widgets/markdown/streaming_markdown_widget.dart

@@ -4,6 +4,9 @@ import '../../theme/theme_extensions.dart';
 import 'markdown_config.dart';
 import 'markdown_preprocessor.dart';
 
+// Pre-compiled regex for mermaid diagram detection (performance optimization)
+final _mermaidRegex = RegExp(r'```mermaid\s*([\s\S]*?)```', multiLine: true);
+
 class StreamingMarkdownWidget extends StatelessWidget {
   const StreamingMarkdownWidget({
     super.key,
@@ -23,8 +26,7 @@ class StreamingMarkdownWidget extends StatelessWidget {
     }
 
     final normalized = ConduitMarkdownPreprocessor.normalize(content);
-    final mermaidRegex = RegExp(r'```mermaid\s*([\s\S]*?)```', multiLine: true);
-    final matches = mermaidRegex.allMatches(normalized).toList();
+    final matches = _mermaidRegex.allMatches(normalized).toList();
 
     Widget buildMarkdown(String data) {
       return ConduitMarkdown.buildBlock(