fix(autofix): Stream content with streaming variant, drop thinking_content fallback

The artifact loading card renders the streamed content from `message.content`.
Apply the `streaming` variant to `<Markdown>` and drop the `thinking_content`
fallback — streamed output is in `content`, not `thinking_content`.

Also removes the stale 'Thinking...' sentinel check that is no longer sent
by the backend.

Co-Authored-By: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>

Author: chromy

static/app/components/events/autofix/v3/artifactLoadingDetails.tsx

@@ -39,12 +39,8 @@ export function ArtifactLoadingDetails({
             return null;
           }
 
-          if (block.message.content && block.message.content !== 'Thinking...') {
-            return <Markdown key={index} raw={block.message.content} />;
-          }
-
-          if (block.message.thinking_content) {
-            return <Markdown key={index} raw={block.message.thinking_content} />;
+          if (block.message.content) {
+            return <Markdown key={index} raw={block.message.content} variant="streaming" />;
           }
 
           return null;