feat: enhance markdown image handling with customizable builder

- Introduced an `imageBuilderOverride` parameter in the `ConduitMarkdown` class to allow customization of how markdown images are rendered.
- Updated the `StreamingMarkdownWidget` to accept the new `imageBuilderOverride` parameter, enabling enhanced image handling in streaming contexts.
- Implemented an `imageBuilderOverride` in the `_AssistantMessageWidgetState` to utilize `EnhancedImageAttachment`, providing caching, authentication headers, and fullscreen viewing for markdown images.
- Refactored the `_ImageBuilder` class to support the new image building logic, improving flexibility and maintainability of image rendering in markdown content.

lib/shared/widgets/markdown/streaming_markdown_widget.dart

@@ -13,11 +13,14 @@ class StreamingMarkdownWidget extends StatelessWidget {
     required this.content,
     required this.isStreaming,
     this.onTapLink,
+    this.imageBuilderOverride,
   });
 
   final String content;
   final bool isStreaming;
   final MarkdownLinkTapCallback? onTapLink;
+  final Widget Function(Uri uri, String? title, String? alt)?
+  imageBuilderOverride;
 
   @override
   Widget build(BuildContext context) {
@@ -34,6 +37,7 @@ class StreamingMarkdownWidget extends StatelessWidget {
         data: data,
         onTapLink: onTapLink,
         selectable: false,
+        imageBuilderOverride: imageBuilderOverride,
       );
     }