fix(chat): restore windowed turn alignment

Keep the virtual-window measurement wrapper as a flex column so AgentTurn alignSelf rules remain active. Add regression coverage for the alignment context.

src/components/chat/AgentWorkspace.test.tsx

@@ -216,6 +216,27 @@ describe("AgentWorkspace", () => {
     expect(renderCounts.size).toBeLessThan(40);
   });
 
+  it("preserves the flex alignment context around windowed turns", () => {
+    const messages = Array.from({ length: 41 }, (_, index): Message => ({
+      id: `message-${index}`,
+      role: index % 2 === 0 ? "user" : "assistant",
+      content: `message ${index}`,
+    }));
+
+    render(
+      <AgentWorkspace
+        {...defaultProps}
+        isStreaming={false}
+        messages={messages}
+      />,
+    );
+
+    expect(screen.getByTestId("turn-message-0").parentElement).toHaveStyle({
+      display: "flex",
+      flexDirection: "column",
+    });
+  });
+
   it("keeps visible turns mounted when crossing the windowing threshold", () => {
     const messages = Array.from({ length: 41 }, (_, index): Message => ({
       id: `message-${index}`,

src/components/chat/AgentWorkspace.tsx

@@ -254,7 +254,12 @@ const MeasuredTurn = ({
   return (
     <Box
       ref={rowRef}
-      sx={{ mb: measure ? `${TURN_GAP_PX}px` : 0, flexShrink: 0 }}
+      sx={{
+        display: "flex",
+        flexDirection: "column",
+        flexShrink: 0,
+        mb: measure ? `${TURN_GAP_PX}px` : 0,
+      }}
     >
       {children}
     </Box>