fix: guard migrations against missing tables and fix highlighted text assertion

- Make migrate_v6_to_v7 check skills table existence before ALTER
- Make migrate_v7_to_v8 check model_pricing table existence before UPDATE
- Fix SessionManagerPage test: use getByRole heading instead of getAllByText
  which breaks when highlightText splits text across <mark> elements

tests/components/SessionManagerPage.test.tsx

@@ -184,7 +184,9 @@ describe("SessionManagerPage", () => {
     });
 
     await waitFor(() =>
-      expect(screen.getAllByText("Alpha Session")).toHaveLength(2),
+      expect(
+        screen.getByRole("heading", { name: "Alpha Session" }),
+      ).toBeInTheDocument(),
     );
 
     fireEvent.click(screen.getByRole("button", { name: /删除会话/i }));