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/features/chat/widgets/assistant_message_widget.dart

@@ -771,6 +771,16 @@ class _AssistantMessageWidgetState extends ConsumerState<AssistantMessageWidget>
       content: processedContent,
       isStreaming: widget.isStreaming,
       onTapLink: (url, _) => _launchUri(url),
+      imageBuilderOverride: (uri, title, alt) {
+        // Route markdown images through the enhanced image widget so they
+        // get caching, auth headers, fullscreen viewer, and sharing.
+        return EnhancedImageAttachment(
+          attachmentId: uri.toString(),
+          isMarkdownFormat: true,
+          constraints: const BoxConstraints(maxWidth: 500, maxHeight: 400),
+          disableAnimation: widget.isStreaming,
+        );
+      },
     );
 
     final responseBuilder = ref.watch(assistantResponseBuilderProvider);